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Reflective Microscope Objectives
LMM15XP01 450 nm  20 µm, LMM25XUVV160 200 nm  20 µm, Application Idea Reflective objective mounted in a Each Finite Conjugate 40X Objective Related Items Please Wait
Applications
Click to Enlarge Click Here for Raw Data The reflectance plot above shows the reflectance of the coated mirrors incorporated into our reflective microscope objectives. Please note that the data shown is per surface, i.e. for one reflection only. Light that enters the objective undergoes two reflections, which decreases the overall energy throughput from that shown here. Features
Thorlabs' Reflective Microscope Objectives consist of reflective surfaces that focus light without introducing chromatic aberration. We offer objectives with two broadband reflective coatings, three magnifications, and in either infinitycorrected or finite conjugate versions. Based on the classical Schwarzschild design, these objectives are corrected for thirdorder spherical aberration, coma, and astigmatism, and have negligible higherorder aberrations, resulting in diffractionlimited performance. Please refer to the Wavefront Error tab above for more details. These advantages make reflective objectives well suited for applications that require longer working distances than those provided by typical refractive objectives. These objectives are available with one of two reflective coatings: a UVenhanced aluminum coating for >80% absolute reflectance in the 200 nm  20 µm wavelength range or a protected silver coating for >96% absolute reflectance in the 450 nm  20 µm wavelength range. Note that this reflectance is for a single coated surface and incoming light is reflected by two coated surfaces. Refer to the tables below for additional specifications. These objectives are RMS threaded (0.800"36) for compatibility with most manufacturers' microscopes. As shown in the images above, the housings are engraved with the part number for easy identification. Additionally, Thorlabs also offers the M32RMSS thread adapter to convert RMS threads to M32 x 0.75 threads. Each reflective objective incorporates a small convex secondary mirror that is mounted on three straight spider vanes. Also referred to as legs, these are visible at the tip of the objective in the middle photo below. Please note that this mirror and the spider vanes act as a central obscuration (blocked area; refer to the images below) that causes a reduction of the contrast for low to mid spatial frequencies. The spider vanes also cause a faint diffraction pattern, which can be evened out through the use of curved spider vanes. For custom objectives with curved spider vanes, please contact Tech Support. Please refer to the Obscuration tab for more information. Alternatively, our Zemax files can provide more details on the effect of the obscuration. Cleaning and Storage Click to Enlarge Each 15X and 25X objective is shipped inside a storage case composed of an OC22 Canister and OC2RMS Lid. 40X objectives shipped with protective caps, which may be substituted with the same storage case (sold separately). Click to Enlarge The frontmounted convex mirror is held by three straight spider vanes. Click to Enlarge The images above were taken with the reflective objective at various offsets from the focal plane. When the spot is accurately focused (0 µm offset in the diagram above), the obscuration effect disappears within the central Airy disc and cannot be resolved any longer. Click to Enlarge The wavefront is partially obscured by the secondary mirror and spider vanes before being reflected by the primary mirror and then the secondary mirror. DiffractionLimited and Minimal Spherical Aberration, Coma, and Astigmatism The intensity maps at the bottom right of each screenshot also show the obscured area in the center of the imaging system. The obscurations are caused by the convex mirror and the spider vane assembly holding the convex mirror in place. Please note that the wavefront and aberration performance of these objectives is consistent regardless of the type of reflective coating, and thus the images and data below apply to the protected silvercoated (P01) objectives as well as the UVenhancedaluminumcoated objectives (UVV). The type of mirror coating primarily affects the light throughput at a given wavelength. Click to Enlarge LMM15XUVV Interferogram with Measured Seidel Coefficients Click to Enlarge LMM25XUVV Interferogram with Measured Seidel Coefficients Click to Enlarge LMM40XUVV Interferogram with Measured Seidel Coefficients
Click to Enlarge Figure 1: The wavefront is partially obscured by the secondary mirror and spider vanes before being reflected by the primary mirror and then the secondary mirror. Effects of ObscurationReflective objective designs incorporate a secondary mirror that is typically mounted on three spider vanes. The mirror and vanes create an obstruction to the entrance pupil (see figures 1 and 2) that decreases transmitted light and modifies the diffraction pattern. Our Zemax files provide more detailed simulation data on the effects of the obscuration of our reflective objectives. Click on the red Document icon () next to the item numbers below to access the Zemax file download. Our entire Zemax Catalog is also available. Central ObscurationThe central obscuration is caused by the convex secondary mirror (green mirror in Figure 2 below) of the Schwarzschild objective. The value specified for the obscuration is the ratio of the obscured area to the entire area of the entrance pupil. If the entrance pupil is homogenously filled, the transmitted light would be reduced by the same factor as the area obscuration. Furthermore, the central obscuration in the entrance pupil results in a redistribution of the intensities of the diffraction rings. Compared to the point spread function of an unobscured system, the central obscuration causes a slightly smaller diameter of the central Airy disc, and thus a slightly higher resolution. However, the secondary maxima also increase in intensity as illustrated in Figure 3 below. The redistribution of the diffraction intensities also affects the contrast of the image, which is evident in the Modulation Transfer Functions (MTF) shown in Figure 4. Compared to an unobscured optical system, the MTF for low and mid spatial frequencies will decrease as the central obscuration increases in size. Also, the MTF increases slightly at high spatial frequencies, which correlates with the slightly smaller central Airy disc (see Figure 3). Click to Enlarge Figure 2: This ray trace diagram illustrates the optics and obstructions in a reflective objective with three straight spider vanes. Click to Enlarge Figure 3: This irradiance graph includes point spread functions and measured data of a gaussian beam through a reflective objective with 26% obscuration. The xaxis is the normalized distance from the theoretical maximum to the first minimum of the reflective objective. Click to Enlarge Figure 4: Obscurations impact the modulation transfer fuction (MTF) and image contrast. The theoretical MTF shown here is for a central circular obscuration with three straight spider vanes. Spider Vane ObscurationBesides the central obscuration, the amount and distribution of the diffracted energy is dependent on the width, shape, and number of spider vanes used to support the secondary mirror. The diffraction pattern in Figure 5 is produced by three straight spider vanes, like those illustrated in figure 2 above. The objectives sold on this page incorporate three straight vanes. Please observe that the plots are given with a logarithmical intensity scale over five orders of magnitude for a better visualization of the diffraction effects. Figure 6 illustrates a diffraction pattern produced by three curved spider vanes. It can be seen that the amount of the diffracted energy will almost be the same, however, with a more even distribution. If this pattern is desirable, our reflective objectives can be customordered with curved vanes by contacting Tech Support. For comparison purposes, Figure 7 presents the pattern resulting from the use of four straight vanes. Click to Enlarge Figure 5: Diffraction Effect from Three Straight Spider Vanes Click to Enlarge Figure 6: Diffraction Effect from Three Curved Spider Vanes Click to Enlarge Figure 7: Diffraction Effect from Four Straight Spider Vanes When viewing an image with a camera, the system magnification is the product of the objective and camera tube magnifications. When viewing an image with trinoculars, the system magnification is the product of the objective and eyepiece magnifications.
Magnification and Sample Area CalculationsMagnificationThe magnification of a system is the multiplicative product of the magnification of each optical element in the system. Optical elements that produce magnification include objectives, camera tubes, and trinocular eyepieces, as shown in the drawing to the right. It is important to note that the magnification quoted in these products' specifications is usually only valid when all optical elements are made by the same manufacturer. If this is not the case, then the magnification of the system can still be calculated, but an effective objective magnification should be calculated first, as described below. To adapt the examples shown here to your own microscope, please use our Magnification and FOV Calculator, which is available for download by clicking on the red button above. Note the calculator is an Excel spreadsheet that uses macros. In order to use the calculator, macros must be enabled. To enable macros, click the "Enable Content" button in the yellow message bar upon opening the file. Example 1: Camera Magnification Example 2: Trinocular Magnification Using an Objective with a Microscope from a Different ManufacturerMagnification is not a fundamental value: it is a derived value, calculated by assuming a specific tube lens focal length. Each microscope manufacturer has adopted a different focal length for their tube lens, as shown by the table to the right. Hence, when combining optical elements from different manufacturers, it is necessary to calculate an effective magnification for the objective, which is then used to calculate the magnification of the system. The effective magnification of an objective is given by Equation 1:
Here, the Design Magnification is the magnification printed on the objective, f_{Tube Lens in Microscope} is the focal length of the tube lens in the microscope you are using, and f_{Design Tube Lens of Objective} is the tube lens focal length that the objective manufacturer used to calculate the Design Magnification. These focal lengths are given by the table to the right. Note that Leica, Mitutoyo, Nikon, and Thorlabs use the same tube lens focal length; if combining elements from any of these manufacturers, no conversion is needed. Once the effective objective magnification is calculated, the magnification of the system can be calculated as before. Example 3: Trinocular Magnification (Different Manufacturers) Following Equation 1 and the table to the right, we calculate the effective magnification of an Olympus objective in a Nikon microscope: The effective magnification of the Olympus objective is 22.2X and the trinoculars have 10X eyepieces, so the image at the eyepieces has 22.2X × 10X = 222X magnification. Sample Area When Imaged on a CameraWhen imaging a sample with a camera, the dimensions of the sample area are determined by the dimensions of the camera sensor and the system magnification, as shown by Equation 2.
The camera sensor dimensions can be obtained from the manufacturer, while the system magnification is the multiplicative product of the objective magnification and the camera tube magnification (see Example 1). If needed, the objective magnification can be adjusted as shown in Example 3. As the magnification increases, the resolution improves, but the field of view also decreases. The dependence of the field of view on magnification is shown in the schematic to the right. Example 4: Sample Area Sample Area ExamplesThe images of a mouse kidney below were all acquired using the same objective and the same camera. However, the camera tubes used were different. Read from left to right, they demonstrate that decreasing the camera tube magnification enlarges the field of view at the expense of the size of the details in the image.
Damage Threshold Data for Thorlabs' Reflective ObjectivesThe specifications to the right are measured data for the mirrors used in Thorlabs' reflective microscope objectives. Damage threshold specifications are constant for all objectives with a given coating type, regardless of magnification or other specs.
Laser Induced Damage Threshold TutorialThe following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratchdig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial. Testing MethodThorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications. The photograph above is a protected aluminumcoated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm^{2} (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.
According to the test, the damage threshold of the mirror was 2.00 J/cm^{2} (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances. Continuous Wave and LongPulse LasersWhen an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) [1]. Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions. When pulse lengths are between 1 ns and 1 µs, laserinduced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating. Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF. In order to use the specified CW damage threshold of an optic, it is necessary to know the following:
Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below. The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other nonuniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right). Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm): While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application. Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information. Pulsed LasersAs previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values. Pulses shorter than 10^{9} s cannot be compared to our specified LIDT values with much reliability. In this ultrashortpulse regime various mechanics, such as multiphotonavalanche ionization, take over as the predominate damage mechanism [2]. In contrast, pulses between 10^{7} s and 10^{4} s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.
When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:
The energy density of your beam should be calculated in terms of J/cm^{2}. The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e^{2} beam. Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately [3]. A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm^{2} at 1064 nm scales to 0.7 J/cm^{2} at 532 nm): You now have a wavelengthadjusted energy density, which you will use in the following step. Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT [4]. For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects. The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1  100 ns, an approximation is as follows: Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10^{9} s and 10^{7} s. For pulses between 10^{7} s and 10^{4} s, the CW LIDT must also be checked before deeming the optic appropriate for your application. Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information. [1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998). In order to illustrate the process of determining whether a given laser system will damage an optic, a number of example calculations of laser induced damage threshold are given below. For assistance with performing similar calculations, we provide a spreadsheet calculator that can be downloaded by clicking the button to the right. To use the calculator, enter the specified LIDT value of the optic under consideration and the relevant parameters of your laser system in the green boxes. The spreadsheet will then calculate a linear power density for CW and pulsed systems, as well as an energy density value for pulsed systems. These values are used to calculate adjusted, scaled LIDT values for the optics based on accepted scaling laws. This calculator assumes a Gaussian beam profile, so a correction factor must be introduced for other beam shapes (uniform, etc.). The LIDT scaling laws are determined from empirical relationships; their accuracy is not guaranteed. Remember that absorption by optics or coatings can significantly reduce LIDT in some spectral regions. These LIDT values are not valid for ultrashort pulses less than one nanosecond in duration. A Gaussian beam profile has about twice the maximum intensity of a uniform beam profile. CW Laser Example However, the maximum power density of a Gaussian beam is about twice the maximum power density of a uniform beam, as shown in the graph to the right. Therefore, a more accurate determination of the maximum linear power density of the system is 1 W/cm. An AC127030C achromatic doublet lens has a specified CW LIDT of 350 W/cm, as tested at 1550 nm. CW damage threshold values typically scale directly with the wavelength of the laser source, so this yields an adjusted LIDT value: The adjusted LIDT value of 350 W/cm x (1319 nm / 1550 nm) = 298 W/cm is significantly higher than the calculated maximum linear power density of the laser system, so it would be safe to use this doublet lens for this application. Pulsed Nanosecond Laser Example: Scaling for Different Pulse Durations As described above, the maximum energy density of a Gaussian beam is about twice the average energy density. So, the maximum energy density of this beam is ~0.7 J/cm^{2}. The energy density of the beam can be compared to the LIDT values of 1 J/cm^{2} and 3.5 J/cm^{2} for a BB1E01 broadband dielectric mirror and an NB1K08 Nd:YAG laser line mirror, respectively. Both of these LIDT values, while measured at 355 nm, were determined with a 10 ns pulsed laser at 10 Hz. Therefore, an adjustment must be applied for the shorter pulse duration of the system under consideration. As described on the previous tab, LIDT values in the nanosecond pulse regime scale with the square root of the laser pulse duration: This adjustment factor results in LIDT values of 0.45 J/cm^{2} for the BB1E01 broadband mirror and 1.6 J/cm^{2} for the Nd:YAG laser line mirror, which are to be compared with the 0.7 J/cm^{2} maximum energy density of the beam. While the broadband mirror would likely be damaged by the laser, the more specialized laser line mirror is appropriate for use with this system. Pulsed Nanosecond Laser Example: Scaling for Different Wavelengths This scaling gives adjusted LIDT values of 0.08 J/cm^{2} for the reflective filter and 14 J/cm^{2} for the absorptive filter. In this case, the absorptive filter is the best choice in order to avoid optical damage. Pulsed Microsecond Laser Example If this relatively longpulse laser emits a Gaussian 12.7 mm diameter beam (1/e^{2}) at 980 nm, then the resulting output has a linear power density of 5.9 W/cm and an energy density of 1.2 x 10^{4} J/cm^{2} per pulse. This can be compared to the LIDT values for a WPQ10E980 polymer zeroorder quarterwave plate, which are 5 W/cm for CW radiation at 810 nm and 5 J/cm^{2} for a 10 ns pulse at 810 nm. As before, the CW LIDT of the optic scales linearly with the laser wavelength, resulting in an adjusted CW value of 6 W/cm at 980 nm. On the other hand, the pulsed LIDT scales with the square root of the laser wavelength and the square root of the pulse duration, resulting in an adjusted value of 55 J/cm^{2} for a 1 µs pulse at 980 nm. The pulsed LIDT of the optic is significantly greater than the energy density of the laser pulse, so individual pulses will not damage the wave plate. However, the large average linear power density of the laser system may cause thermal damage to the optic, much like a highpower CW beam.
Click to Enlarge The diagram above illustrates the working distance and parfocal distance for reflective objectives with infinite back focal length.
These reflective objectives are designed with infinite back focal length and are ideal components of an infinitycorrected optical system in combination with our tube lenses. We offer three magnifications as well as two different reflective coatings (see table below for details). They are RMS threaded (0.800"36) for compatibility with most manufacturers' microscopes. Click on the red Document icon () next to the item numbers below to access the Zemax file download. Our entire Zemax Catalog is also available.
Click to Enlarge The diagram above illustrates the working distance, parfocal distance, and back focal length for reflective objectives with a finite back focal length.
These reflective objectives are designed with a finite back focal length of 160 mm and are ideal for imaging applications where no refractive optical elements are desired. We offer three magnifications as well as two different reflective coatings (see table below for details). They are RMS threaded (0.800"36) for compatibility with most manufacturers' microscopes. The LMM40XUVV160 and LMM40XP01160 objectives are shipped with an attached PLE152 Parfocal Length Extender. This hollow extender increases the parfocal length to 45 mm to match other parfocal length standards, such as those used by Olympus and Leica. Click on the red Document icon () next to the item numbers below to access the Zemax file download. Our entire Zemax Catalog is also available.
 
