Response-Flattening Filters for Silicon Photodiodes

  • Flattens Spectral Response over 400 - 1100 nm
  • Ø1/2" and Ø1" Unmounted Options


Ø1/2" Flattening Filter for Si Photodiodes


Ø1" Flattening Filter for Si Photodiodes

SRF11 Mounted onto the Housing of a DET36A2 Silicon High-Speed Photodetector Using an SM1 Lens Tube

Application Idea

The SRF15 and SRF11 filters improve the non-uniform response of silicon photodiodes, resulting in a shaped responsivity curve. The plot shows the calculated spectral response of an FDS100 photodiode with and without the filter.

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Item #  SRF15 SRF11
Size Ø1/2" Ø1"
Spectral Range 400 - 1100 nm
Clear Aperture >90% of Diameter
Surface Flatness λ/2 at 633 nm
Over Clear Aperture
λ/4 at 633 nm
Over Clear Aperture
Surface Quality 40-20 (Scratch-Dig)
Parallelism ≤3 arcmin
Transmission Data
Raw Data
Thickness 1.0 mm (0.04") 3.2 mm (0.13")
Thickness Tolerance ±0.25 mm +0.00 / -0.10 mm
Diameter Ø1/2" (12.7 mm) Ø1" (25.4 mm)
Diameter Tolerance +0.00 / -0.25 mm +0.00 / -0.10 mm
Substrate Fused Silica
Coating Dielectric Coating on Front Surface
Cleaning Tutorial


  • Designed to Flatten Spectral Response of FDS100 and Other Si Photodiodes
  • Performance over 400 - 1100 nm Wavelength Range:
    • 10% Minimum Transmission at 985 nm
    • 95% Maximum Transmission at 400 nm
  • Facilitates Power Monitoring When Used with Broadband / Tunable Light Sources
  • Reduces Dynamic Range Requirements for Post-Photodiode Electronics
  • Ø1/2" and Ø1" Unmounted Sizes

The filters on this page feature a dielectric coating on fused silica and are designed to compensate for the wavelength-dependent responsivity of silicon photodiodes. Improving the response uniformity is beneficial for applications where a broad spectral response is desired. These filters are suitable for reducing dynamic range requirements for detector system electronics and can be used to improve the performance of applications dependent on feedback from optical power monitoring. The filter is designed to reshape the responsivity rather than to attenuate the light. We suggest combining the response-flattening filter with neutral density filters or edgepass filters when attenuation is required.

These filters are designed to be used with the FDS100 photodiode as well as the SM05PD1A and SM05PD1B mounted photodiodes and DET36A(/M) detector. However, they can also be used with silicon photodiodes in general, with varying degrees of spectral responsivity flattening depending upon the specific responsivity of the diode. To calculate the expected flattened spectral response induced by the filter, multiply together the transmission of the filter and spectral response of the Si photodiode. The graph above shows the calculated flattening effect for the FDS100 Si photodiode. Refer to the table to the right to download the transmission data. When using with the DET36A(/M), the bias voltage and amplifiers of the detector may alter the effective responsivity of the diode. Note that, depending upon the distance of the filter from the photodiode and the angle of the filter with respect to the normal, multiple reflections may occur that will alter the transmission through the filter; adjust the filter appropriately to account for this effect. Thorlabs' wedge prism mounting shims can be used to accommodate back reflections at different wedge angles.

The SRF11 Ø1" filter can be mounted onto the external SM1 (1.035"-40) threads of the DET36A(/M) using an internally threaded SM1 lens tube and an SPW606 SM1 spanner wrench. To mount the SRF15 Ø1/2" filter to the detector, use an SM05 lens tube, an additional SM05RR retaining ring, and an SPW603 SM05 spanner wrench. Alternatively, the filters can be mounted using our wide selection of filter mounts.

Posted Comments:
Sebastian Marschall  (posted 2022-03-04 08:31:13.22)
Are considering to offer powermeter heads equipped with a response flattening filter? For instance, an analog to the S116C, S120C or S140C would be very interesting. Best regards, S. Marschall
wskopalik  (posted 2022-03-09 10:43:57.0)
Thank you very much for your feedback! I will contact you directly to talk about your application in further detail. Technically such a sensor is most likely possible as a custom. But we would need to check your requirements to see how well such a sensor would work. E.g. thermal power meter sensors have a flat response curve as well and are usually a good alternative.
Sebastian Marschall  (posted 2021-09-14 03:00:17.637)
Hello! Do you have data for the angular dependence of the filter transmission curve? Or, within what range of the angle of incidence does the filter work according to specification? Best regards, S. Marschall
YLohia  (posted 2021-10-15 11:36:29.0)
Hello Marschall, thank you for contacting Thorlabs. Unfortunately, we only have data for 0˚ AOI. The filter transmission shifts a negligible amount for small AOIs (+/- 6˚). This allows the end-user to reject multiple reflections at a small angle. If you need a part that corrects for the response of a different photodiode, the entire filter will need to be redesigned. If that's the case, please contact your local Thorlabs Tech Support group (in your case, with your requirements.
ulrich.leischner  (posted 2018-03-12 09:43:52.153)
Is this filter working at 370nm? can they be used at wavelength down to 350nm? We are using a Hamamatsu S1337-33BQ Photodiode. Regards Dr. Ulrich Leischner
nbayconich  (posted 2018-03-16 01:45:42.0)
Thank you for contacting Thorlabs. Unfortunately, we do not have transmission data for the SRF11 or SRF15 below 400 nm, because it's too close to 100% transmission and too oscillatory to be reliable.
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+1 Qty Docs Part Number - Universal Price Available
SRF15 Support Documentation
SRF15Ø1/2" Response Flattening Filter for Silicon Photodiodes, 400 - 1100 nm
SRF11 Support Documentation
SRF11Ø1" Response Flattening Filter for Silicon Photodiodes, 400 - 1100 nm