I want to photograph rooms and spaces indoors, and covered areas outdoors, and get good measurements of illumination. Light sources will be sun, sky, and artificial. Another use is to photograph materials side by side with a variety of reflectivities, to get accurate measures of those reflectivities.

I can handle the physics - watts per steradian square meters and all that. I just need a camera where I can be sure pixel values are proportional to physical illumination - no built-in gamma correction or curves or other enhancements etc.

I could use RAW but I'd prefer to use ordinary formats for smaller size. Of coure 8-bit/channel formats will give me only 256 distinct values; I can live with that, since I can widely bracket exposures. There is no motion to be concerned about.

Which off the shelf cameras are most suitable for this use? Or alternatively, how to test a given camera for linearity and accuracy?

  • I'd like to create tags "photometry" and "calibration", maybe "scientific usefulness" or something, but I have too low points at this time! – DarenW Aug 2 '10 at 5:09
  • Many camera's now have an sRAW format, that might be a happy compromise. As you are probably aware, the reason why ordinary formats are smaller, is due to compression, so no matter how neutral the resultant jpeg is, there will still be dataloss on certain pixels of the image. – Alan Aug 2 '10 at 5:49

It sounds like you need a scientific imaging device. I was told when I worked with these things that scientific grade CCD imaging devices are the most linear devices known to man, in contrast to the imagers discussed by @Guffa. I'm talking about cameras made by photometrics, pco (the sensicam), or devices made for astrophotography or microscopy.

These imagers are distinct from commercial grade imaging devices in that:

  • No lens. You have to supply that; this is a pure detector. The mount is typically C or F mount.
  • There are no hot pixels or cold pixels (at least in the $20k/chip range). If there are, return to the manufacturer for a replacement.
  • A few years back, 1280x1024x8fps was considered very good. Maybe they've gotten larger since then, I don't know.
  • You can bin (combine pixels to increase the sensitivity of the device, and decrease the spatial resolution).
  • The logic for reading pixels from the device is very good. On older (over ten years) devices, there was a slight error when moving pixel values from one pixel to the next to read out the value at the Analog/Digital converter at the edge of the chip. That error is essentially zero in modern devices. Contrast this with CMOS imagers, where the readout happens on each pixel (and so the A/D conversion may not be the same from pixel to pixel).
  • The chip is cooled, usually to -20 to -40 C, so as to minimize noise.
  • Part of the manufacturer's specification is the Quantum Efficiency, or the percentage chance that a photon will be converted to an electron and recorded. A backthinned CCD might have a QE of around 70-90% for a green (450nm) photon, whereas others might be more in the 25-45% range.
  • These imagers are pure black and white, recording a spectrum that is indicated by the manufacturer and can go into the IR and UV ranges. Most glass will cut UV (you have to get special glass or quartz to let it pass), but IR will probably need some more filtering.

The sum of these distinctions means that the value of each pixel correlates very highly with the number of photons that struck the physical location of the pixel. With a commercial camera, you have no guarantees that pixels will behave the same as one another (and in fact, it's a good bet that they don't), or that they behave the same way from image to image.

With this class of device, you'll know the exact amount of flux for any given pixel, within the boundaries of noise. Image averaging then becomes the best way to handle noise.

That level of information may be too much for what you want. If you need to go commercial grade, then here's a way to go:

  • Get a Sigma imaging chip (Foveon). These were originally made for the scientific imaging market. The advantage of this chip is that each pixel is red, green, and blue overlapping each other, rather than using a Bayer sensor, where the pixel pattern is not overlapping.
  • Use this camera only at iso 100. Don't go to the other iso's.
  • Place the camera in front of a light source of known output at a known distance. The flatter this illumination (ie, goes from edge to edge of the camera), the better.
  • Record images at a given exposure time, and then either modify the exposure time to change the apparent flux at the sensor, or change your light source.
  • From this set of images, create a curve that shows the average pixel value in red, green, and blue for a known flux. That way, you can translate pixel intensity to flux.
  • If you had a completely flat illumination profile, you can also describe the behavior of your lens viz edge dropoff.

From here, you can take a picture of a room (or something else) in controlled conditions where you know what the answer is and validate your curves.

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  • ummm..... it's a relief to know I can get a linear sensor free of hot pixels for only $20k/chip... uh, I guess I'll be paying closer attention to the second half of your answer. It would be a fine way to spend an hour one weekend to make this calibration curve. Finding a light source of known output may be an interesting task in itself. – DarenW Aug 3 '10 at 6:23
  • Yeah, it turns out that photon counting isn't a very easy thing to do outside of a lab environment. One light source of known output would be a laser pointer; that should be pretty stable, known wattage, etc. Trying to flatten a laser output across the whole image might be interesting, maybe by using a fogged mirror or something. – mmr Aug 3 '10 at 17:24

I think that most cameras would work for this, provided that they produce RAW (or DNG) files and that they have manual exposure settings.

If you don't use the RAW format, the image will be processed. This usually means that some curve is applied, and it always means that you lose some information. The RAW format usually has higher data resolution (e.g. 12 bits per pixel instead of 8), and the JPEG compression throws away a lot of information.

I don't think that you can get a completely linear result out of any camera, the chip is simply not designed with a completely linear response as the most important aspect. So, you still would need an adjustment curve to translate the pixel values into luminance values. You can photograph a grayscale to determine the response for each tone.

You should use manual settings in the camera to get a consistent result. You can have different settings for different amount of light, but as the response is not completely linear, I think that you need a separate adjustment curve for each setting.

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If you must shoot JPEG, make sure the camera has good customizable image settings. Turn contrast down, and any kind of highlight or shadow correction off.

For example, on my camera, if I shoot Natural mode with Contrast-4, Sharpness-4, it is close to linear.See if you can ask dpreview how their tests are made, or just go through all of their reviews as they do have tone curves. From what I gather, most other manufacturers (in my class) do not allow linear uncompensated highlights to the extent of the Pentax. Look at the link under Dynamic Range compared and Contrast

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