I have taken a bunch of photos using a forensic bite mark scale; perfect size to put in photos with the study organism – fungi fruit bodies. It is a commonly used one that you can buy pretty cheaply (e.g. https://www.lociforensics.nl/products/crime-scene-documentation/rulers-photo-scales/abfo-no-2—bitemark-scale). It states in the info for this tool that the reflectance value is about 18, I would like to know if there is a way to double check this somehow? I will be calibrating these photos to extract CIELAB values. I can take RAW photos with this standard and the X-Rite ColorChecker Passport in the same photo, but I have no idea if it is possible to measure reflectance from this. Or do I need to ask around the departments and see if someone has a spectrometer I can use? If push comes to shove, can I use the value of 18 and then double check the photo output somehow? I am just made a bit unsure by the use of ‘about 18%’ in the product information.
Firstly, if you’ve used the same standard in your entire dataset, then to some degree it doesn’t matter too much how accurate the reflectance is. You’ll still be getting high precision calibrated images, with perhaps all the numbers one or two percentage points above or below their true values (it’s all relative, so rarely matters too much). However, it does matter if you need absolute accuracy (e.g. between studies).
That said, you can measure it. It’s easier to measure reflectance with a camera than a spectrometer unless you really know what you’re doing. Simply take a photo of your ~18% standard next to a standard of known reflectance (e.g. an x-rite colour checker with known grey values – the values are mentioned in this forum and are in the toolbox). Calibrate the image against the master standard and then measure the unknown standard in this image.
Those bite-mark scales look like an excellent design. How diffuse is the grey on them though?
Thanks for the fast answer! Great to know that I can measure it with the camera and the photos I already have – that will make life much easier. I will probably measure them anyway as although this data isn’t going to be used for other studies at the moment, I am hoping to put the values in a trait database for later use. I will post here the values I get for the two standards I have (two teams were using them) in case it is of use to anyone in the future.
Yes, they are a really handy design and good to have in field as they are pretty robust and the scale bars on them are excellent. A bit weird ordering something from a forensic lab that is normally used in police cases, but it does the job. In terms of difuseness, they seem pretty good – they are pretty matte and I can only get light reflecting off them if I hold at certain angles in direct sunlight, which is almost never the case for me as I work in shady forests (this may be a problem if you were working in grasslands or similar though). I think because they are designed to calibrate photos that are used as in evidence in court, they have to be to a pretty high level. Not as good as something like a Spectralon standard of course, but the field work budget didn’t stretch that far 🙂
Thanks again, Sam