#19 + #2007 mounted together.

[Pall Tamas]

This is going to be either reinvention of the wheel or the biggest BS you heard but I am still going to ask: 

what happens if I put a #19 and a #2007 in front of the lens? 

Theoretically the 2 together should result in an almost clean IR over 720nm with a very small contamination (under 10%) arround 580-600nm. 

Transmission @740nm would be 90% of 70% (63%) which is bad but clean, or at least cleaner than the one or the other alone. 

what am I missing? 

Thanks in advance

Tom

[Chris Fastie]

Tom,

You might not be missing anything, but I will reserve judgement. If you were stranded on a desert island, or anywhere else, and needed to take a pure near infrared photo to save your life, or my life, or anybody's life, putting a blue filter (#2007) and a red filter (#19) in front of a full spectrum camera (no IR block filter) might very well make you a hero. Very little visible light can pass through both filters, but lots of NIR can. Here are spectral curves for a blue filter (BG3) and red filter (Wratten 25) which are very similar to the #2007 and #19.

Both filters have near zero transmission for (almost overlapping) parts of the visible range, so you cannot see much light when you hold both up to a light bulb. But a full spectrum camera would make a nice NIR photo through both filters. If you wanted more light to reach the sensor, you could use a Wratten 87 filter instead of the two colored filters, but many desert islands don't have Wratten 87, so hip hip hooray, Tom.

Chris

The KAPtery

[Pall Tamas]

Great! Many thanks Chris! 

 Then I will test the results tomorrow against a Hoya R72. 

You are right about me being on an island it's called Hungary and there is no wratten filter here. 

I have the 19 and 2007 already and i am a couple weeks away from getting my Micasense Sequoia so I really don''t want to put too much in this camera.

Chris, another question: 

ho should i set the white balance as i don't have a target?

[Chris Fastie]

The Hoya R72 is almost the same thing as a Wratten 87. So that will give better results than the red and blue filters stacked. 

For pure NIR photos there is no need for a special white balance. With a Hoya R72 on the camera, all three channels will capture only NIR.

A custom white balance setting helps if you are using a red or blue filter to make photos for uncalibrated NDVI.

Chris

[Pall Tamas]

Chris, 

do you measure incident light or use reflectance target? 

[Chris Fastie]

When a red or a blue filter is used (one at a time) on a full spectrum camera to make photos that can produce uncalibrated NDVI, a custom white balance can be set while filling the frame with strongly colored light. We have found that for certain red filters, filling the frame with a piece of bright red paper in direct sunlight does an acceptable job. All this does is fool the camera into exaggerating the relative brightness of the blue channel which is where NIR is captured. That way values used for NIR are bigger than those used for red and NDVI computes to something like real NDVI.