Beyond Visible
Monochromatic Ultraviolet & Infrared Testing of the Nikon D70

Shane Elen

Simple Monochromatic Ultraviolet Testing
A Nikon D70 was used in a darkroom to record monochromatic ultraviolet light of 1nm band pass. This is extremely narrow band light of one specific wavelength e.g. 350nm +/-0.5nm. The image was defocused to try and diffuse the light a little and the image intensities are not an indication of CCD response. This is because the xenon source used does not have a uniform output but contains numerous spikes superimposed on a broadband emission. A series of images were captured in steps of 5nm for light below 405nm and testing was stopped when no image could be obtained by the camera. This end point might be beyond "normal lighting" situations but it is an indication of the range possible when a very strong ultraviolet source of this wavelength is used. The purpose was to determine the ultraviolet range of this camera when fitted with different two lenses, the El-Nikkor 63mm f3.5 and the UV-Nikkor 105mm f4.5. However, it also revealed another interesting characteristic with regards to how the Nikon D70 CCD deals with ultraviolet light.

Nikon D70 with El-Nikkor 63mm f3.5
This combination was fitted with the 18A ultraviolet transmission filter (300-400nm) which also has an infrared transmission region. Under some test conditions could bias the results. However the advantage of monochromatic light is that absolutely no infrared is present in the light source and therefore all images are pure ultraviolet.

335nm
Fig. 335nm 340nm
Fig. 340nm (extremely weak) 345nm
Fig. 345nm 350nm
Fig. 350nm 355nm
Fig. 355nm

335nm
Fig. h335nm 340nm
Fig. h340nm 345nm
Fig. h345nm 350nm
Fig. h350nm 355nm
Fig. h355nm

360nm
Fig. 360nm 365nm
Fig. 365nm 370nm
Fig. 370nm 375nm
Fig. 375nm 380nm
Fig. 380nm

360nm
Fig. h360nm 365nm
Fig. h365nm 370nm
Fig. h370nm 375nm
Fig. h375nm 380nm
Fig. h380nm

385nm
Fig. 385nm 390nm
Fig. 390nm 395nm
Fig. 395nm 400nm
Fig. 400nm 405nm
Fig. 405nm

385nm
Fig. h385nm 390nm
Fig. h390nm 395nm
Fig. h395nm 400nm
Fig. h400nm 405nm
Fig. h405nm

Nikon D70 fitted with the UV-Nikkor 105mm f4.5
The distribution of ultraviolet light into the RGB channels is the same as for the El-Nikkor above, however the range extends down to 320nm.

320nm
Fig. u320nm 325nm
Fig. u325nm 330nm
Fig. u330nm 335nm
Fig. u335nm 340nm
Fig. u340nm

320nm
Fig. hu320nm 325nm
Fig. hu325nm 330nm
Fig. hu330nm 335nm
Fig. hu335nm 340nm
Fig. hu340nm

RGB channel distribution of ultraviolet

It was observed from the histograms of these images that the blue channel was dominant for ultraviolet light between 395nm and 400nm, whereas the red channel became dominant for 390nm and below. In general the green channel was the weakest channel however between 380nm and 370nm it contained enough signal to be useful. Useful for what? In situations where infrared is contaminating the ultraviolet image it might be possible (Nikon D70) to exclude the red channel and improve the ultraviolet contrast by using either the blue and/or green channel data only. Of course this will only work if the sample actually reflects ultraviolet in this range, otherwise the channel will have no data in it - check the histogram. For more information on dealing with infrared contamination go here

The following image was captured using an ultraviolet-free infrared flash and the histogram exhibits the distribution of infrared throughout the blue, green and red channels. The infrared data is concentrated primarily in the red channel, however some also occurs in the green channel and the least amount occurs in the blue channel. By comparing the ultraviolet and infrared histograms it is evident that some overlap can exist when both forms of light are present. This illustrates why infrared contamination of an ultraviolet image can be a problem. More detailed infrared distribution into the RGB channels can be found below.

Histogram of an infrared image
Fig.16. Histogram for the infrared dandylion image in fig. 17 Infrared image of a dandylion
Fig.17. Infrared dandylion image

Simple Monochromatic Infrared Testing
The following was obtained with a Nikon D70 fitted with a UV-Nikkor 105mm f4.5 lens and a Hoya 695nm filter. The data was collected using the same method as for the ultraviolet data above.

700nm
Fig. 700nm 725nm
Fig. 725nm 750nm
Fig. 750nm 775nm
Fig. 775nm