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Cube-Mounted Pellicle Beamsplitters

  • No Ghosting or Beam Displacement
  • No Chromatic Aberration with Uncollimated Beams
  • SM1 and 30 mm Cage System Compatible




Michelson Interferometer
with Pellicle Beamsplitter

Application Idea

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General Specifications
Ports 4, all SM1 Threaded (1.035"-40)
Membrane Material Nitrocellulose
Housing Material Engraved Black Anodized Aluminum
Membrane Thickness 2 µm, 5 µm for 300-400 nm Version
Temperature Range -40 to 70 °C
Index of Refraction 1.5 at 550 nm
Surface Quality 40-20 Scratch-Dig
Transmitted Wavefront Error 0.5λ (Typical)
Reflected Wavefront Error 1λ (Typical)
Item # (Click to View) Wavelength Splitting Ratio
CM1-BP108 Uncoated,
400 nm - 2.4 µm
CM1-BP145B5 300 - 400 nm 45:55
CM1-BP145B1 400 - 700 nm 45:55
CM1-BP133 635 nm 33:67
CM1-BP150 635 nm 50:50
CM1-BP145B2 700 - 900 nm 45:55
CM1-BP145B3 1 - 2 µm 45:55
CM1-BP145B4 3 - 5 µm 45:55
Cube Mounted Pellicle Beamsplitter with Port Hole Cover
Click to Enlarge
CM1-BP145B3 Shown with Two SM1CP2 Port Hole Covers (Sold Separately)


  • Eliminates Ghosting
  • No Chromatic Aberration with Focused Beams
  • Minimal Change in Optical Path Length
  • SM1-Threaded Entrance and Exit Ports
  • 30 mm Cage System Compatible

Thorlabs' Ø1" mounted pellicle beamsplitters are ideal for use in applications where chromatic dispersion must be minimized (i.e. cases where focused beams are necessary). These beamsplitters virtually eliminate ghosting since the second surface reflection is superimposed on the first one. However, they are extremely fragile due to the nitrocellulose membrane being microns thick, which exhibits less than 1/2 wave of variation at 635 nm across its 25 mm diameter. To provide maximum protection from damage, these beamsplitters are housed inside a 30 mm cage-system-compatible cube. The cubes are post-mountable and have SM1-threaded access ports, making them compatible with our entire line of Ø1" lens tubes and accessories. The cubes are M6 x 0.5 threaded, but include 8-32 and M4 mounting adapters. These Beamsplitter cubes can also be connected to other cage cubes with cage rods and our ERSCA adapters.

Please Note: Pellicle beamsplitters exhibit sinusoidal oscillations in the splitting ratio as a function of wavelength, due to thin film interference effects. See the Thin Film Tutorial tab for more details. These sinusoidal oscillations can be clearly seen in the plots below.

The engraving on the top of the mounts indicates the direction of incident light. This direction is consistent with the light being incident upon the coated surface of the beamsplitter first.

Thorlabs offers three types of mounted beamsplitters: Non-Polarizing Beamsplitting Cubes, Polarizing Beamsplitting Cubes, and the Pellicle Beamsplitters presented below. A large variety of unmounted beamsplitters are also available. For a direct comparison of the performance of our non-polarizing beamsplitting cube, plate, and pellicle at 633 nm, see the Lab Facts tab.

For a complete selection of our cube-mounted optics please see the Mounted Optics Guide tab.

Handling Precautions
Our pellicle beamsplitters are manufactured from an extremely thin and fragile membrane. Please do not touch the membrane under any circumstances. Compressed or canned air should also not be used on these beamsplitters, as the force of the air is large enough to damage the membrane. If the pellicle becomes damaged, please contact tech support for information about replacement of the beamsplitter; in these cases, we only charge for the cost of the pellicle.

Thorlabs Lab Fact: Beamsplitter Package Matters

We present laboratory measurements of the polarization angle, split ratio, and total throughput power of a beam transmitted through Thorlabs plate, cube, and pellicle beamsplitters. While all non-polarizing beamsplitters function similarly, the exact performance is different for different types of beamsplitter. Each type of beamsplitter contains its own advantages and disadvantages compared to other types of beamsplitters. Appropriate choice of beamsplitter is essential to sensitive experimental systems. We present a complete analysis and comparison of optical parameters for three common types of non-polarizing beamsplitters.

For our experiment we used the former generation HRS015 stabilized HeNe laser (replaced by the HRS015B) as the light source for our investigation. A linear polarizer is used to set the laser beam's polarization axis to 45° in order to provide equal S- and P-polarized light incident on the beamsplitter. The beamsplitter under investigation was then placed in the beam path, and its split beams directed to appropriate detectors. The total power though the optic, polarization states, split ratios, and angle of incidence effects were investigated under this configuration.

The plots below summarize the measured results for all three types of beamsplitters. From these graphs the performance of each optic can be easily compared to one another. The bottom left plot summarizes the results for the total power throughput for each optic. The total power throughput is measured as the fraction of input power. While the plate and pellicle beamsplitters perform rather similarly, the cube shows signs of absorption inside the optic. Additionally, this plot shows the relative insensitivity of throughput power to angle of incidence. The bottom middle graph summarizes the results for the output polarization angle for each optic. The cube shows the most similar polarization angles between the reflected and transmitted beams, with the plate producing the largest difference in polarization between beams. The bottom right plot summarizes the results for the split ratio, as a fraction of input power, for the beamsplitters. Here it can be shown that the plate beamsplitter demonstrates the most ideal for 50/50 power splitting. For details on the experimental setup employed and the results summarized here, please click here.

Legend for Beam Diagrams

Reflected Beam: reflected

Transmitted Beam: Transmitted Beam (Green)

Beamsplitter Selection Guide

Thorlabs offers six main types of beamsplitters: Pellicle, Cube, Plate, Polarizing Bandpass, Economy, and Polka Dot. Each type has distinct advantages and disadvantages.


Pellicle Beamsplitter

Pellicle Beamsplitters - Pellicle beamsplitters are the best choice when dispersion must be kept to a minimum. They virtually eliminate multiple reflections commonly associated with thicker glass beamsplitters, thus preventing ghosting. In addition, unlike plate beamsplitters, there is a negligible effect on the propagation axis of the transmitted beam with respect to the incident beam.

Pellicle beamsplitters have two disadvantages: They exhibit sinusodial oscillations in the splitting ratio as a function of wavelength, due to thin film interference effects. Click Here for more details. They are also extremely delicate. Since they are fabricated by stretching a nitrocellulose membrane over a flat metal frame, the beamsplitter cannot be touched without destroying the optic. Thorlabs offers pellicle beamsplitters mounted in metal rings for use in kinematic mounts as well as 30 mm cage cube-mounted pellicles.


Polarizing Beamsplitting Cube
Nonpolarizing Beamsplititng Cube

Beamsplitting Cubes
Thorlabs’ beamsplitter cubes are composed of two right-angled prisms. A dielectric coating, which is capable of reflecting and transmitting a portion of the incident beam, is applied to the hypotenuse surface. Since there is only one reflecting surface, this design inherently avoids ghost images, which sometimes occur with plate-type beamsplitters. Antireflection coatings are available on the entrance and exit faces of certain models to minimize back reflections. As well as providing a cost-effective solution, another advantage of the beamsplitting cube is the minimal shift it causes to the path of the transmitted beam. Thorlabs offers both polarizing and nonpolarizing beamsplitting cubes, in mounted and unmounted configurations. Mounted beamsplitters are available that are compatible with our 16 mm cage systems as well as our 30 mm cage systems.

Polarizing Beamsplitters - Thorlabs’ polarizing plate and cube beamsplitters split randomly polarized beams into two orthogonal, linearly polarized components (S and P), as shown in the diagram to the right. S-polarized light is reflected at a 90° angle with respect to the incident beam while p-polarized light is transmitted. Polarizing beamsplitters are useful in applications where the two polarization components are to be analyzed or used simultaneously. Thorlabs offers broadband 16 mm cage cube-mounted, broadband 30 mm cage cube-mounted, and broadband unmounted polarizing beamsplitter cubes, as well as laser line 30 mm cage cube-mounted and laser line unmounted cubes. Additionally, Thorlabs offers wire grid polarizing beamsplitters which have a larger Angle of Incidence and work with uncollimated light. For applications requiring higher power, we also offer high-power polarizing beamsplitting cubes.

Non-Polarizing Beamsplitting Cubes - These cubes provide a 50:50 splitting ratio that is nearly independent of the polarization of the incident light. The low polarization dependence of the metallic-dielectric coating allows the transmission and reflection for s- and p-polarization states to be within 10% or 15% of each other. These beamsplitters are particularly useful with randomly polarized lasers and are specifically designed for applications in which polarization effects must be minimized. Thorlabs offers 16 mm cage cube-mounted, 30 mm cage cube-mounted, and unmounted beamsplitter cubes.


Plate Beamsplitters

Plate Beamsplitters - Thorlabs' plate beamsplitters are optimized for an incidence angle of 45° and feature a dielectric coating on the front surface for long-term stability. To help reduce unwanted interference effects (e.g., ghost images) caused by the interaction of light reflected from the front and back surfaces of the optic, a wedge has been added to the round versions of these beamsplitters. Dispersion, ghosting, and shifting of the beam may all be potential problems, however. These are the best choice for a general-purpose beamsplitter. Thorlabs offers both polarizing and nonpolarizing plate beamsplitters.


Plate Beamsplitters

Polarizing Bandpass Filters - Thorlabs' Polarizing Bandpass Filters are designed to isolate key laser lines, such as Nd:YAG, HeNe, and diode, while also separating out the s- and p-polarization states. The p-polarized component is transmitted over a defined pass band and reflected (rejected) outside of the band, while the s-polarized component is reflected over the entire blocking region of the optic. They are designed to be used at a 45° AOI; however, when used at the center wavelength, the incident angle can be widened without loss of performance.


Economy Beamsplitters

Economy Beamsplitters - These are the most cost effective of all the beamsplitter types. Thorlabs' economy beamsplitters, which have an exposed oxide coating on one side and are uncoated on the other side, are designed to have either a 50:50 or 30:70 splitting ratio throughout the visible spectrum (450 - 650 nm) when used with unpolarized light incident at 45°.

Please note that the Fresnel reflections off of the uncoated back surface of these economy beamsplitters can lead to interference effects in the reflected beam. For applications sensitive to these effects, consider using a beamsplitting cube or a pellicle beamsplitter.


Polka Dot Beamsplitters - This type of beamsplitter consists of a glass substrate with a vacuum-deposited reflective coating that is applied over an array of apertures, giving the beamsplitter a "polka dot" appearance. Half of the incident beam is reflected from the coating, and half of the beam is transmitted through the uncoated portion of the substrate.

Polka Dot Beamsplitter

Polka dot beamsplitters are useful over a wide wavelength range and are negligibly angle sensitive, which makes them ideal for splitting the energy emitted from a radiant source. These are not recommended for imaging applications, such as interferometry, as the polka dot pattern will affect the image.

30 mm Cage-Cube-Mounted Optics Selection Guide

The table below provides links to all of our 30 mm Cage-Cube-Mounted optics. For our selection of 16 mm Cage-Cube-Mounted Optics, please see our 16 mm Cage Systems guide.

Non-Polarizing Beamsplitters Polarizing Beamsplitters High Power Polarizing Beamsplitters
Non-Polarizing Beamsplitter Cube Polarizing Beamsplitter Cube High-Power Polarizing Beamsplitter Cube
Pellicle Beamsplitter Laser Line Polarizing Beamsplitters Circular Polarizer
Pellicle Beamsplitters Laser Line Polarizing Beamsplitter Cube Circular / Variable Polarizers
Penta Prisms Turning Mirrors Variable Beamsplitters/Attenuators
Penta Prisms Turning Mirrors Variable Beamsplitters / Attenuators

30 mm Cage Cube Empty Optic Mounts Selection Guide

Dichroic Mirror Holder Empty Compact Cage Cube
Rectangular Dichroic Mirrors and Filters Empty Compact 30 mm Cage Cube

Thin-Film Interference Effect

Reflectance Curve showing Thin-Film Interference Effects

The plots below, which show transmission as a function of wavelength, depict a sinusoidal fluctuation that is caused by interference effects. These effects will occur for all pellicle beamsplitters and are sometimes averaged out when data is displayed.

The graph to the right depicts the actual measured percent reflectance of an 8:92 beam sampler without averaging the sinusoidal oscillations that result from thin film interference. The frequency and amplitude of the pattern depends on four factors: the thickness of the film, the thickness of any coating present, the angle of incidence of the incoming light, and the polarization of the incoming light.

Q: What is a thin film?
A layer of material is referred to as a thin film if the thickness of the layer is on the order of the wavelength of incident radiation in the film medium. The relationship between the wavelength of light in air and that in the film is given by

Relationship between wavelength in air and film

For the pellicle beamsplitters featured here, the information under the Specs tab states that nfilm = 1.5 at 550 nm. Therefore, for this incident wavelength, the wavelength in the pellicle membrane itself is

Wavelength of 550 nm light in pellicle beamsplitter

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Posted Comments:
Posted Date:2015-06-24 11:03:00.127
I have a beam splitter CM1-BP108 uncoated. My question is, what is the damage threshold if I have a collimated beam with a diameter of 3 mm. I would like to us it for powers up to 5 W at around 1100 nm. Thanks
Posted Date:2015-08-18 12:24:13.0
Response from Bweh at Thorlabs USA: The uncoated pellicle will withstand that power level, cw or pulsed. Note that localized heating of the film may change the thickness slightly causing a change in the R/T. Changing the incidence angle will help tune the R/T ratio if critical for application.
Posted Date:2013-06-24 12:20:28.693
This looked to be what I needed, then looked at the cad drawing. The beamsplitter is circular and needs to be elliptical so that there is full beam transmission through the BS. Look at reflector telescopes to see the usefulness of an elliptical mirror, it has a circular shape when viewed at 45 degrees. Steve
Posted Date:2013-07-11 16:25:00.0
Response from Tim at Thorlabs: Thank you for your feedback. We do carry elliptical mirrors but currently all of our pellicles are circular. I am checking into our ability to provide this and I will contact you to keep you updated.
Posted Date:2013-01-04 09:44:40.02
Response from Jeremy at Thorlabs: If the pellicle inside is damaged, we can take back the cube-mounted pellicle for repair. In such a case, we will only charge the price of the replacement pellicle.
Posted Date:2012-12-21 17:28:21.01
I am worried about the lifetime of cube mounted Pellicle beam splitter. I agree they are better protected, however they are also 4 times more expensive than the unmounted type. Is it possible to purchase an unmounted type an insert it into the cube as a replacement if/when needed?
Posted Date:2012-07-05 14:03:00.0
Response from Tim at Thorlabs: We currently do not have empirical data for the CM1-BP145B1 LIDT. However, we recommend not exceeding 2W/cm^2 for these beamsplitters. Please use this as a reference, but note that other factors, as described in our LIDT tutorial at, will vary this number.
Posted Date:2012-07-05 15:21:35.0
Can you provide the damage threshold of CM1-BP145B1 (pellical beamsplitter), for cw laser? Thanks.
Posted Date:2012-05-01 19:07:00.0
Response from Tim at Thorlabs: Thank you for your feedback. You are correct that these are averaged graphs and that individual parts will have oscillations from interference. We note this in the footnote as you mentioned and also provide a “Tutorial” tab on the Pellicle Beamsplitters page describing this interference effect. If you would like to see an example of this curve for a specific part number, please contact us at so we may provide you with this data.
Posted Date:2012-05-01 11:32:42.0
The thin film interference referred to in the footnote to the graphs can be excessive. The graphs are from averaged data that hides these oscillations. Ask for a representative graph prior to purchase.

8:92 (R:T) Cube-Mounted Pellicle Beamsplitter, Uncoated: 400 - 2400 nm

Click Here for Raw Data for the 250 – 2500 nm Wavelength Range

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
CM1-BP108 Support Documentation
CM1-BP108Cube-Mounted Pellicle Beamsplitter, 8:92 (R:T), Uncoated, 400 - 2400 nm

45:55 (R:T) Cube-Mounted Pellicle Beamsplitter, Coating: 300 - 400 nm

Click Here for Raw Data for the 250 – 2500 nm Wavelength Range

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
CM1-BP145B5 Support Documentation
CM1-BP145B530 mm Cage Cube-Mounted Pellicle Beamsplitter, 45:55 (R:T), 300 - 400 nm

45:55 (R:T) Cube-Mounted Pellicle Beamsplitter, Coating: 400 - 700 nm

Click Here for Raw Data for the 250 – 2500 nm Wavelength Range

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
CM1-BP145B1 Support Documentation
CM1-BP145B130 mm Cage Cube-Mounted Pellicle Beamsplitter, 45:55 (R:T), 400 - 700 nm
Lead Time

33:67 (R:T) Cube-Mounted Pellicle Beamsplitter, Coating: 635 nm

Click Here for Raw Data for the 250 – 2500 nm Wavelength Range

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
CM1-BP133 Support Documentation
CM1-BP13330 mm Cage Cube-Mounted Pellicle Beamsplitter, 33:67 (R:T), 635 nm

50:50 (R:T) Cube-Mounted Pellicle Beamsplitter, Coating: 635 nm

Click Here for Raw Data for the 250 – 2500 nm Wavelength Range

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
CM1-BP150 Support Documentation
CM1-BP15030 mm Cage Cube-Mounted Pellicle Beamsplitter, 50:50, 635 nm

45:55 (R:T) Cube-Mounted Pellicle Beamsplitter, Coating: 700 - 900 nm

Click Here for Raw Data for the 250 – 2500 nm Wavelength Range

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
CM1-BP145B2 Support Documentation
CM1-BP145B230 mm Cage Cube-Mounted Pellicle Beamsplitter, 45:55 (R:T), 700 - 900 nm

45:55 (R:T) Cube-Mounted Pellicle Beamsplitter, Coating: 1.0 - 2.0 µm

Click Here for Raw Data for the 250 – 2500 nm Wavelength Range

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
CM1-BP145B3 Support Documentation
CM1-BP145B330 mm Cage Cube-Mounted Pellicle Beamsplitter, 45:55 (R:T), 1 - 2 µm

45:55 (R:T) Cube-Mounted Pellicle Beamsplitter, Coating: 3.0 - 5.0 µm

Click Here for Raw Data for the 0.25 – 20.0 µm Wavelength Range

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
CM1-BP145B4 Support Documentation
CM1-BP145B430 mm Cage Cube-Mounted Pellicle Beamsplitter, 45:55 (R:T), 3 - 5 µm

Cage Cube Connector

CM1-CC Application
Click to Enlarge
Two CM1-CC Connectors used to Connect Multiple 1.5" Wide Cage Cubes
  • Connect Two 1.5" Wide Cage Cubes Side by Side
  • Compatible with CM1 and CCM1 Series Cage Cubes

The CM1-CC cube connector allows two or more cubes to be connected as shown in the image to the right. Many of our cage cubes are compatible with this connector, including empty cubes, empty dichroic cubes, mounted beamsplitters, mounted penta prisms, and mounted turning mirrors.

Two cage cube-mounted turning mirrors cannot be connected using the CM1-CC due to a lack of Ø6 mm cage rod holes on two sides of the cube.

Alignment Pins
Please note that because dowel alignment pins are used, the connector requires drilled holes on the cube face between the SM1-threaded (1.035"-40) ports. If you have an older cube and would like it updated to have alignment holes for free, please contact Technical Support. Alternatively, the alignment pins are press-fit inside their mounting holes, and can be pressed out for use with cubes that do not have these alignment holes.

Based on your currency / country selection, your order will ship from Newton, New Jersey  
+1 Qty Docs Part Number - Universal Price Available / Ships
CM1-CC Support Documentation
CM1-CCCage Cube Connector for Compact 30 mm Cage Cubes
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