Wednesday, September 06, 2017

On the Use of White Light Source for Imager Spectral Response Measurements

Aphesa publishes a blog post "Why You Can't Use A White (Broad Spectrum) Light Source For Your Spectral Measurements." Few quotes:

"The R, G, B or the monochrome curve is not the response curve of the sensor but only some information about it.

First of all, it is usually only measured for an incident beam perpendicular to the sensor, or in other words at a zero chief ray angle. But we know that the spectral response varies in intensity and in shape with the angle of incidence.

Secondly, it is a noisy measurement and therefore the precision depends on the number of samples taken. The measurement is noisy because the amount of light after the monochromator is limited as only a tiny fraction of the source's spectrum reaches the sensor. Therefore the signal level is small and its SNR is therefore low.

The shape of the spectrum also depends on the bandwidth of the measuring instrument. As the response spectrum can exhibit oscillations at several scales (see this other publication about spectral response), only a very small bandwidth, i.e. a narrow monochromator lid, can reproduce the actual shape of the response curve, any other approach will only produce a smoothed curve without any detail.

Finally, the wide band light source will change over time and its spectrum will change over temperature, therefore requiring regular calibration.

The light source that we use is based on a femtosecond laser and a supercontinuum.

The femtosecond laser is a solid laser that provided very high power light pulses with a duration in the range of the femtosecond. Lasers are known to provide coherent, repeatable and high intensity light. Our laser is red and the laser has the size of a small table.

The role of the supercontinuum is to turn the monochromatic laser light into a wide spectrum by a collection of non-linear processes. The supercontinuum is a long microstructured optical fiber. The fiber seems red at its beginning and is white in the end as the spectrum broadens along the fiber.
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10 comments:

  1. Can you share a result comparison between a well done monochromator SR measurement and one with the laser (done on the same sensor)?

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    1. I unfortunately only have one sensor measured with the two approaches and it was not measured in exactly the same conditions. The measurement is also confidential.

      What we see is:
      - for the same target accuracy and CRA, the measurement time is less because we need to average less frames
      - it is possible to reach 1nm spectral resolution with reasonable performance (other methods may be very inaccurate in deep blue and NIR)

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    2. Albert Theuwissen - Harvest ImagingSeptember 9, 2017 at 2:20 PM

      Be aware that if you have two measurements with a different result, you never know which one is correct and which one is not correct. There is a nice paper written by people of Berkeley about measuring QE with an accuracy of over 99 %. That is something you do not do in an afternoon and is costing you a LOT of effort.
      Another remark : I think you mix two measurements. Namely spectral response and angular dependency. spectral response should/can be measured with perpendicular incoming light (chief ray = 0 deg.), it is always done that way. And indeed, then you get the best/optimal results. Next you can characterize the angular dependency.

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    3. Hi Albert,
      Can you supply reference for the paper you mentioned?

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    4. Albert,

      indeed QE is measured perpendicular, but angular dependency is also a function of wavelength and therefore it needs to compare full curves and not only a few wavelengths.

      The chosen solution is to provide full spectral and angular data as a matrix and any plot can come out of it.

      Color constancy of a camera also depends on how the QE curves vary in the corners compared to the center due to the change in angle of incidence.

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  2. Arnaud, are you actually telling us that the EMVA1288 equipment you developed and that you sell is not using the right illumination for the QE measurements ?

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    1. The laser method is compatible with the method described in section 9.1 of EMVA1288 as it is still a broadband light source with a wavelength selection. It is only better than the usual method used.

      There is an draft appendix to EMVA1288 for the working group only that is related to the angular dependency and F/# dependency of the results and in that one we need more light power.

      As for everything else in EMVA1288, the standard is made such that a better setup gives you better results but the standard works with a minimal setup (we have built a compliant setup for 1.3 euro).

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    2. Can you elaborate more of what you did for 1.3 euro ?!

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  3. Here you go..
    http://snap.lbl.gov/ccdweb/EI2006_Max_LBNL59378.pdf

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    1. Albert Theuwissen - Harvest ImagingSeptember 13, 2017 at 8:17 AM

      Thanks, this is indeed the reference I was referring to !
      These guys proof the accuracy of the measurement method that is in use for decades.

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