Photometric measurements of a laptop monitor

This post completes the path from the journey from light intensity, or exposure, to pixels on the screen. In the last one I looked at how my Nikon D300 translates intensities in the scene being photographed into values in the RAW and JPEG files. The response of the RAW file is very linear, doubling the exposure (or equivalently intensity) doubles the value in the pixel in the RAW file. The response in the JPEG file is more complex; over a 4-stop range the value in the file increases as the log of the exposure. The final part of the story is how do these values in the file correspond to intensity on a computer monitor. As seen last week, RAW files from a digital camera are a good method of measuring intensity.. this is what is called photometry (at least in the field of astronomy).

The first interesting measurement that can be made with a D300 is the colour temperature (CT) of the monitor. One way to do this is to just take a bunch of JPEG shots of a white window on your monitor, with the white balance set to various colour temperatures. The image in which the Red, Green and Blue channels look the best matched is the one that had the correct colour temperature setting. You can see this on the D300 colour graphs, or offline using a program like photoshop. Some measured curves are below:

The photo with 4350K CT looks too blue, showing that the monitor is hotter than that. Similarly the 7140K shot is dominated by the red channel, indicating the CT is less than 7140K. The best match between all channels is at a colour temperature between 5500K and 5880K. Interestingly, according to Wikipedia the sRGB standard demands a CT of 6500K, so my monitor is evidently a little below the standard.

The second thing that can be measured is the intensity of the light generated by the monitor for a know intensity in an image file. The testcard I used, which was drawn with GIMP (similar to photoshop) is shown immediately below, and the photograph of how it appeared on my monitor is below it. To save space on the blog, the JPEG version of the photo is shown, but I used the RAW file for the photometry, since its response is linear.


The relationship between value in the testcard and intensity on the monitor is plotted below.

The response is somewhat linear over an approximately 5-stop range. However, even when taken on a dark evening (with the lights off!), the monitor does not go completely dark, which presumably means the LCD display is only "so good" at blocking the fluorescent backlight from shining through.

The characteristic curves of the camera and display can be put together to give an overall response, as shown below. The results are rather depressing, they make me feel like I should get a better display!

I would be interested to see how other displays respond.. would a really expensive display be more linear and go darker -- presumably a CRT could go completely dark. It's my impression that my old laptop had a better display than my newer one, I'll have to test it some time.