Camera sensors are developed for recreating images in the human-visible range, however the sensors themselves (CMOS or CCD) are actually typically sensitive down into the relevant UVA range for modelling animal vision (typically about 320nm), and the same sensors are extremely sensitive to the near-infrared range (up to about 900nm). Almost all cameras have a filter directly in-front of the sensor which blocks out the “unwanted” UV and IR wavelengths. Removal of the filter results in a “full spectrum” camera, which is predominantly sensitive to IR and human-visible wavelengths, but also to the critical UV range. Now filters can be put infront of the lens to photograph in the spectral regions of interest (typically visible and UV).
In order to image in the UV range a filter is required which blocks out all IR and visible-spectrum light. It is essential that there is absolutely no IR leakage in the UV filter! This can be tested by photographing an IR light source and seeing whether you can see anything in the image (e.g. a television remote control), also healthy plant foliage should appear dark in UV (but bright in IR) when photographed under natural light conditions.
Does the filter need replacing with a new full-spectrum filter?
Many camera-conversion specialists will offer to place a filter back over the sensor which either has full-spectrum transmission (e.g. quartz, which is fine), or will block out some portion of the spectrum, such as UV-only (it is essential to avoid anything like this – these methods require a camera which is sensitive to both UV and the visible-range).
However, these additional filters are not at all necessary, and may even increase internal reflections in the camera (reducing dynamic range). They primarily offer mechanical protection for the sensor (so just be careful when cleaning the sensor and you’ll be fine without one of these filters). The other function these filters can provide is restoring the refractive properties of the camera system given the designed back-focal-length of the lens. So it is possible that, for example, focussing to infinity might not be possible if the sensor location is not readjusted. DSLRs and mirrorless cameras have sensors mounted on adjustable plates, which can typically be used to compensate fully for this effect.
Where to acquire a full-spectrum camera?
|Ready-converted||Ready-converted full spectrum cameras can be bought online. e.g. check sites such as eBay using the search term “full spectrum UV camera”. They are used by astrophotographers, forensics experts, and (ridiculously) for ghost hunting. Note that if you see a “ghost” in your UV images, but not your visible-range image, this is due to internal reflections in the lens, which are often more extreme in the UV and IR ranges because the lens coatings are not optimised for these wavelengths.|
|Have your own camera converted||A number of businesses sell camera conversion as a service. e.g. ACS (empirical imaging has no affiliation with this company).|
|DIY conversion||Some cameras can be converted manually with nothing but a small screwdriver (though note that there is a very real possibility of damaging the camera – some are very tricky to take apart – empirical imaging is not responsible for any damage you may cause to your camera). Search the internet for a “teardown” of your camera model. Here is an example of the Samsung NX1000 (which even given its age, is an excellent UV camera).|
|Sony Alpha Range||These are excellent mirrorless cameras. Mirrorless systems are actually preferable over standard DSLRs for full-spectrum photography because they perform the metering based on sensor measurements (rather than other optical systems which are not able to work in the UV range). Mirrorless cameras therefore tend to have better automatic exposure abilities in the UV range. The Sony A7 range is an excellent choice, though the cropped-sensor versions will also be suitable.|
|Samsung NX range||As above, but these can often be bought extremely cheap second hand. So that you might feel more confident attempting to do a DIY full-spectrum conversion.|