Setting White Balance with IR filters

[Manfred M]

More old information that may not be relevant to current technology, Paul.

"The new innovation in the D3/D300 autoexposure system is the introduction of a small diffraction grating or Diffractive Optical Element (DOE for short) in front of the autoexposure lens. The DOE bends the light passing through it by an amount proportional to its wavelength, or color. The amount of this bending is precisely controlled, such that the red, green, and blue wavelengths are shifted by an amount equal to the spacing of the columns of pixels in the RGB sensor."

Nikon had been doing this type of metering for a generation or two before Canon adopted it. Canon was being roundly criticized about a decade ago for using monochrome metering.

[dje]

More old information that may not be relevant to current technology, Paul.

"The new innovation in the D3/D300 autoexposure system is the introduction of a small diffraction grating or Diffractive Optical Element (DOE for short) in front of the autoexposure lens. The DOE bends the light passing through it by an amount proportional to its wavelength, or color. The amount of this bending is precisely controlled, such that the red, green, and blue wavelengths are shifted by an amount equal to the spacing of the columns of pixels in the RGB sensor."

This appears to be an alternative to individual colour filters on the pixels. There is a diagram in the F5 reference (post 35) which shows the different colour pixels in columns. More speculation but interesting!

Dave

[pnodrog]

More old information that may not be relevant to current technology, Paul.

"The new innovation in the D3/D300 autoexposure system is the introduction of a small diffraction grating or Diffractive Optical Element (DOE for short) in front of the autoexposure lens. The DOE bends the light passing through it by an amount proportional to its wavelength, or color. The amount of this bending is precisely controlled, such that the red, green, and blue wavelengths are shifted by an amount equal to the spacing of the columns of pixels in the RGB sensor."

Thanks that made the penny drop. (anybody know that expression?) I had assumed when they put "RGB" preceding "sensor" it implied a bayer filter in place. In the above description it seems far more likely that the sensor is unfiltered. It must be the PR people adding the term RGB in front of the word sensor as I doubt an engineer would. An unfiltered silicon sensor's response extends beyond RGB. A little into the ultraviolet and fairly sensitive in the NIR region. Interestingly such an arrangement using a diffraction element and an unfiltered sensor could easily be adapted in design for correct exposure evaluation with IR filters.

[dje]

Great (?) minds think alike Paul. You would probably need an infra red cutoff filter with this arrangement to prevent the IR spreading over into the adjacent blue pixels in the next column.

Dave

[xpatUSA]

Great (?) minds think alike Paul. You would probably need an infra red cutoff filter with this arrangement to prevent the IR spreading over into the adjacent blue pixels in the next column.

Dave

I once was into measuring emission spectra from various types of lamp using a slit and a diffraction grating.



One thing I learned is that the lines per mm affects the angular width of the cast spectrum (and the resolution) and that the grating casts several spectra:

https://opentextbc.ca/universityphys...tion-gratings/

Hopefully the physics haven't changed since the link was published ...

[Manfred M]

Hopefully the physics haven't changed since the link was published ...

I haven't used that type of gear since 1st year university physics. I think yours is probably more sophisticated and a lot cheaper than the lab gear we were using.

[pnodrog]

I once was into measuring emission spectra from various types of lamp using a slit and a diffraction grating.



One thing I learned is that the lines per mm affects the angular width of the cast spectrum (and the resolution) and that the grating casts several spectra:

https://opentextbc.ca/universityphys...tion-gratings/

Hopefully the physics haven't changed since the link was published ...

Beautifully engineered. Almost as good as the very best from Perkin Elmer, Bausch and Lomb or Beckman Instruments.....

[xpatUSA]

Beautifully engineered. Almost as good as the very best from Perkin Elmer, Bausch and Lomb or Beckman Instruments.....

Thank you, Paul!

It works well enough for my purposes. Here's a comparison between it and a commercial lab test for my 5000K LED flood lamps:



I shot the grating end and then ran a plot profile in ImageJ. X-axes may not be perfectly aligned wave-length wise.

So, what was the correct WB for that shot, eh?