Full-spectrum Photography - Basics

Basics

Digital sensors and photographic films can be made to record non-visible ultraviolet (UV) and infrared (IR) radiation. In each case, they generally require special equipment: converted digital cameras, specific filters, highly transmitting lenses, etc. For example, most photographic lenses are made of glass and will filter out most ultraviolet light. Instead, expensive lenses made of quartz must be used. Infrared films may be shot in standard cameras using an infrared pass filters, although focus must compensate for the infrared focal point.

A converted digital camera usually requires that the infrared hot mirror be removed and replaced by a wideband, spectrally flat glass of the same optical path length. Typical glass types used include Schott WG-280 and BK-7, which transmit as much as 90% from around 300 nm to past 1000 nm. Removing the hot mirror is tedious and may require special tools and clean rooms.

Once the camera is sensitive to the full-spectrum, external filters can be used to selectively filter portions of the UV, visible and infrared to achieve various effects. For example, a standard red #25a can be used to include red light and infrared light together, yielding particularly strong two-toned color images of a reddish nature except where the infrared is high and shows as cyan. Another example, using UV/IR filters such as the 18A or U-330 yield a two or three toned image in which blues and yellows dominate. Less common filters have been claimed to give a variety of color effects ranging from diverse pastel foliage and deep blue skies to surrealistic effects of the sky and ground, though digital image processing is likely required to achieve the full effects. One issue with Full-spectrum photography in either film or digital photogrqaphy is the chromatic aberration produced by the wideband information. That is, different spectra, including the ultraviolet and infrared, will focus at different focal points, yielding blurry images and color edge effects, depending on the focal length used. There are specialized lenses such as the Nikon 105mm f4.5 UV-Nikkor which are designed to eliminate this chromatic aberration.

It is important to note that while the converted camera sensor is capable of recording in both the ultraviolet and infrared region, when mixed light hits the sensor it will be the longer infrared waves that will predominate in the recording. Little or no shortwave ultraviolet light may be recorded unless selective filtering is applied to cut some or all of the infrared light. The longwave infrared light may also wash out a considerable amount of the visible light in the blue and green areas in a full spectrum photograph. Similarly if infrared light is entirely blocked, the visible light can overwhelm the recording of the ultraviolet light. So there is no truly full-spectrum photograph that can be made.

Full-spectrum photography achieves various effects and surrealistic colors from the interaction of reflectivity (UV, visible, IR) of nature and man made materials and the specific spectral transmission of the red, green and blue filters on the camera. The addition of external filters will reduce and emphasize different interactions, yielding different effects.