##Background

My digital pictures can be read into a computer program like Matlab or R as a m x n x 3 matrix where m x n is the number of pixels observed by each of the three (red, green, and blue) sensors, and each cell in the matrix has a number from 1-255 that reflects the brightness observed by the sensor.

I would like to use this information to obtain an objective measure of greenness in a photograph, because I want to attempt to correlate greenness to plant growth (imagine one picture per day of a corn field).

Previous work in this direction has had some success by calculating an index of green either as

• green % = green/(blue + red) or
• green divergence = 2*green - red - blue

from webcam images for each of the m x n pixels, but there was no control over the aperture or incident radiation (solar angle).

note that I am not looking for an 'absolute' measure of greenness, the scale and distribution of the number does not matter - it just has to provide a consistent relative measure of greenness.

##Question

Can I use my SLR to get a robust measure of greenness that is invariant with any or all of the following:

• cloud cover?
• time of day?
• day of year? (this is the only requirement)
• proportion of sky / ground in the background?

##Current Status

I have come up with the following ideas, but I am not sure which would be necessary, or which ones would have no effect on the ratio of green/(red + blue)

1. take a picture of a white piece plastic, and use this image to normalize the other values
2. Fix aperture
3. Fix shutter speed
4. set the white balance using a white piece of paper
5. Take all photos from the same angle
6. Take all photos at solar noon

Background

My digital pictures can be read into a computer program like Matlab or R as a m x n x 3 matrix where m x n is the number of pixels observed by each of the three (red, green, and blue) sensors, and each cell in the matrix has a number from 1-255 that reflects the brightness observed by the sensor.

I would like to use this information to obtain an objective measure of greenness in a photograph, because I want to attempt to correlate greenness to plant growth (imagine one picture per day of a corn field).

Previous work in this direction has had some success by calculating an index of green either as

• green % = green/(blue + red) or
• green divergence = 2*green - red - blue

from webcam images for each of the m x n pixels, but there was no control over the aperture or incident radiation (solar angle).

note that I am not looking for an 'absolute' measure of greenness, the scale and distribution of the number does not matter - it just has to provide a consistent relative measure of greenness.

Question

Can I use my SLR to get a robust measure of greenness that is invariant with any or all of the following:

• cloud cover?
• time of day?
• day of year? (this is the only requirement)
• proportion of sky / ground in the background?

Current Status

I have come up with the following ideas, but I am not sure which would be necessary, or which ones would have no effect on the ratio of green/(red + blue)

1. take a picture of a white piece plastic, and use this image to normalize the other values
2. Fix aperture
3. Fix shutter speed
4. set the white balance using a white piece of paper
5. Take all photos from the same angle
6. Take all photos at solar noon