Obtaining intensity/irradiance in an image

[fog37]

TL;DR

Find intensity/irradiance of the pixels in a picture

Hello,

Is it possible to take a picture/image using a digital camera and then obtain the intensity/irradiance ($W/m^2$) of the composing pixels? Should the image be exported in a particular format into a graphics software like photoshop to be able to obtain the irradiance information? Or should some simple code (say Python) be written and applied to the image to access its irradiance information?

Thanks!

 

[hutchphd]

First of all I recommend ImageJ
You can do pretty good photometry using most cameras. The voltage on a pixel is usually quite linear with photon number. One needs to be cognizant of manipulations done by the camera however, both optically and post processing. Also the flatness of the field.

ImageJ allows you to look in detail at the files and manipulate them ad infinitum. It's free from NIH so play with it some. Obviously you want a bitmap format of some kind. Play.

 

[fog37]

Thanks.

I just used ImageJ and I am exploring its functions.
As you mention, an image is made of pixels and the brightness of each pixel (which corresponds to the amount of reflected light) is collected as a electric voltage that should be linear with brightness. Once the image is imported in ImageJ, I wonder if the pixel values must be scaled and multiplied by factor to obtain the energy in Joule at each pixel...

 

[hutchphd]

Yes. On high end (often cooled) astronomical/fluorescence cameras they will often provide you the number (usually the quantum yield i.e. electrons per photon ) for the average pixel and then describe any amplifiers. You will need some way to calibrate your camera...either a good radiometer/photometer for comparison or a known source. One way to do it is to use the moon on a clear night.
Astronomers are often interested in doing these kinds of calibrations so you might research that area. Also there are some good amateur astronomers here. I have never done it for an ordinary camera.

 

[pbuk]

Obviously you want a bitmap format of some kind.

Compression will greatly corrupt the per-pixel information: you need both a camera and software that will handle a RAW image.

 

[fog37]

I see how the data needs to be raw. A raw image is such that each pixels records the amount of light that hits it as a voltage level. Wouldn't a good digital camera provide, in the documentation, the conversion factor between voltage level and light energy on each pixel?

The calibration does not seem trivial...

thanks

 

[fog37]

Yes. On high end (often cooled) astronomical/fluorescence cameras they will often provide you the number (usually the quantum yield i.e. electrons per photon ) for the average pixel and then describe any amplifiers. You will need some way to calibrate your camera...either a good radiometer/photometer for comparison or a known source. One way to do it is to use the moon on a clear night.
Astronomers are often interested in doing these kinds of calibrations so you might research that area. Also there are some good amateur astronomers here. I have never done it for an ordinary camera.

Hello hutchphd, thanks again for bring ImageJ to my attention :) Very easy to use.

I may need one more piece of advice: assuming same illumination, etc. I am take two images of the two similar objects with a digital camera and would to compare the image brightness (mean intensity, integrated intensity, easy to get in ImageJ) in the same ROIs. I can do that.

But before I do the comparison analysis, so the comparison is fair, should I convert the two images from jpg to some other format inside ImageJ? For example, should I first convert both two images to 8-bit before comparing them (Image-> Type-> 8-bit> save)?

Thanks

 

[hutchphd]

It would be better if you could get a non compressed raw bitmap image from your camera (bmp). If jpeg you want the least compression possible and you might want to compare some images at different compression. I do not know exactly what jpeg does. For manipulation just keep it bmp until you are finished. Have fun

 

[fog37]

It would be better if you could get a non compressed raw bitmap image from your camera (bmp). If jpeg you want the least compression possible and you might want to compare some images at different compression. I do not know exactly what jpeg does. For manipulation just keep it bmp until you are finished. Have fun

Thanks!
I will borrow the camera tomorrow (a Canon ). Based on the camera instruction manual, I believe it can only save images either as jpeg or RAW. I guess I would save them as RAW and then import them into ImageJ. Instead of using Open..., I will import them using Import -> Raw..., unless I am making some mistake doing that...