# Question: What is the working principle of Blue/Red filter

by nickyshen0306 |

https://publiclab.org/wiki/infragram#Filters

I got some basic knowledge about the blue/red filter on the above link.

I am confusing how a piece of red glass paper (red filter) can block the blue light, so that the blue channel can use for capture NIR? The red filter would not affect the green channel? If yes, what is the impact?

As I am trying to calculate the ENDVI , so I need to know how the filter affecting green channel. The equation from calculating the ENDVI shown as below:

ENDVI = [(NIR + Green) -- (2 * Blue)] / [(NIR + Green) + (2 * Blue)]

The inexpensive red and blue filters used in DIY single camera NDVI systems typically block most green light. So the camera's green channel captures some NIR and some red or blue but not much green (see this wiki).

Dichroic filters can be designed to pass just about any wavelengths you need, so green light could be captured in the green channel (see this note). Dichroic filters are expensive. This one passes green, red, and NIR. Photos taken with this filter will have red light (and some NIR) in the red channel, green light (and some NIR) in the green channel, and NIR in the blue channel.

A yellow or orange filter can allow mostly green to be captured in the green channel, mostly red in the red channel, and mostly NIR in the blue channel (see this note). An orange filter will allow more visible light to contaminate the blue channel so the NIR data will not be as pure as with a red filter. A Wratten 15 filter might allow a facsimile of ENDVI.

Chris

Thank you for your helping hands!!