What does "Full Spectrum" mean in photography?

Updated: Nov 9

Color temperature and Color Rendering Index (CRI) are the standards for measuring light. There is no technical definition of "full-spectrum" so it cannot be measured.

"Full Spectrum Photography"

The term "full spectrum photography" can be a bit confusing. Does that mean the full visible spectrum? Visible plus IR? Full spectrum plus UV and IR? And do what degree?

Full-spectrum photography is defined among photography enthusiasts as imaging with the full, broad spectrum of a film or camera sensor bandwidth. In practice, specialized broadband/full-spectrum film captures visible and near infrared light, commonly referred to as the "VNIR".

Modified digital cameras can also detect some ultraviolet, all of the visible and much of the near infrared spectrum. Most digital imaging sensors detect from around 350 nm to 1000 nm.

An off-the-shelf digital camera contains an infrared hot mirror filter that blocks most of the infrared and a bit of the ultraviolet that would otherwise be detected by the sensor, narrowing the accepted range from about 400 nm to 700 nm.

How do they make infrared cameras? Pretty much by not blocking these frequencies with a hot mirror filter (which has to be replaced with a wideband glass of the same optical path length), and by enhancing the collection of light in the IR or UV range. It's all about sensors and filters.

Replacing a hot mirror or infrared blocking filter with an infrared pass or a wide spectrally transmitting filter allows the camera to detect the wider spectrum light at greater sensitivity. A converted full-spectrum camera can be used for ultraviolet photography or infrared photography with the appropriate filters.

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.

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.

One issue with full-spectrum photography in either film or digital photography 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. (2)

Thus there is no truly full-spectrum photograph that can be made.

1 https://en.wikipedia.org/wiki/Multispectral_image

2 https://en.wikipedia.org/wiki/Full-spectrum_photography

© 2020 by AGHOST

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