For making objective measurements of reflectance (the amount of light reflected by an object relative to a reflectance standard), colour, and pattern all you need is a digital camera that can produce RAW images (or one you have made a linearisation model for), a good quality lens (e.g. with no vignetting – darkening at the image edges), and a grey or white standard (or, in certain circumstances two or more standards light and dark standards). Most pattern analysis requires a scale bar. Lighting also needs to be considered carefully.
The images produced by this set-up will be objective in terms of measuring reflectance levels, being repeatable with the same equipment, and robust against changes in lighting conditions, but are strictly device-specific. Different camera models have different spectral sensitivities, so measurements made with a different set-up will produce slightly different results. The JPG photographs produced by some equipment may also not be objective until transformed to cone-catch images. This is because of the complex colour spaces the images are converted to.
Equipment check-list for objective measurements
|Cameras which can take RAW photos are best, because this format saves linear, uncompressed pixel values. This includes digital SLRs and mirrorless cameras. However the toolbox has tools which support almost any camera which you have made a linearisation model for (even standard smart phones). The camera must cover the biologically relevant spectral range for your hypothesis (e.g. consider whether you need a UV camera).
|Ideally use a “prime” (i.e. not a zoom) lens. A high quality lens is desirable as this will minimise vignetting (images getting darker towards the corners), radial distortion (e.g. if you photograph a chequer board the lines in the photo should all be perfectly straight), and chromatic distortion.
|Diffuse grey standard(s)
|These can be purchased from most photography suppliers and will specify their own reflectance value. The X-Rite colorChecker passport is convenient as it has a range of grey levels (which can also be used for linearisation modelling and cone-catch modelling). See here for more information on standards.
|A ruler, or some object of known size is required for various imaging techniques (e.g. pattern analysis, acuity control), see the scale bar section.
|It is essential to consider lighting. See the lighting section for more information.
|Colour charts can be used for creating a cone-catch model, and can be used to create a linearisation model for cameras which do not have RAW output. You must know the spectral reflectance of the colours in the colour chart in order to create a cone-catch model. The X-Rite colorChecker Passport data are included with the toolbox.