Broad band VIS+NIR LEDs

Are they useful for photography?


Pedro J. Aphalo






LED light, comparison, VIS, NIR

Broadband VIS + VIS LEDs based on a blue LED and “phosphors” have appeared in the market. The principle is the same as used for most white LEDs, but including also “phosphors” fluorescing at longer wavelengths. One example is type SMB1N-BB450 from Roithner LaserTechnik (Vienna, Austria). The primary LED has a peak of emission at 450 nm and the spectrum as a whole covers the range 400 nm to 1000 nm. Except for the prominent blue peak the spectrum is rather flat, making this LED useful as a light source for VIS and specially NIR spectrometry.

This LED type seems useful for photography as well as for its intended use in spectrometry. Emission is rather weak and efficiency low, emitting 0.3W of radiation from 2.5W of electrical power. At less than 20€ it is not expensive. It has a flatter spectrum than incandescent lamps and emits insignificant amount of thermal radiation. The LED is sold in an SMD package unsoldered, although a suitable PC board is also available. A photograph of the LED is shown below.

SMB1N-BB450 LED soldered to metal-core circuit board

SMB1N-BB450 LED soldered to metal-core circuit board

I did two types of measurements: 1) spectral irradiance measurements of the bare LED and of the LED filtered with yellow, orange, red and near-infrared long pass filters; 2) photographed a colour checker pattern with an of-the-shelf Olympus E-M1 Mk II camera and with a “full-spectrum” converted Olympus E-M1 (“Mk I”) camera. In the case of the full spectrum camera I photographed the colour checker with the bare lens and through most of the same filters filters used for the spectral measurements.

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Figure 1: These five images captured with a full-spectrum converted camera have been white balanced using the grey 18% reflectance patch. The values for the extreme patches of the grey patches row, at the bottom of the colour checker, were edited to approximately the same luminance in all five images. The shorter the cut-in wavelength of the filters the weaker the false colour, as expected.
Figure 2: These images were captured with the of-the-shelf camera and white balanced. Without a custom profile colours in images taken using the SMB1N-BB450 LED are muted. Because of the very strong blue component, white balancing the image results in lots of noise even at ISO 200. White balance settings from grey patch: >14000K, -4.1; 5360K, +2.5; 5340K, +3.2; 5180K, +3.6.

Use of a pale yellow filter with the full-spectrum converted camera helped, and is the subject of the next test. We start with another set of spectra. The Heliopan Yellow 5 filter fluoresces in the VIS and NIR when illuminated with blue light.

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          "Each curve in combination with a different stack of filters") +
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Figure 3: The colour checker illuminated with the VIS+NIR LED with different filters on the full-spectrum converted camera. White balance settings from grey patch: >14000K, -18.5; 12900K, -7.5; 5082K, +11.7; 4700K, +6.5.

Further details: A Datacolor Spyder CHECKR 24 card with 18 colour patches and 6 patches for a grey-scale with known spectral reflectance was used. The objective was a Sigma 30mm 1:1.4 DC DN. The Custom ICC profile was created with Lumariver Profile Designer. The images shown are cropped as the colour checker occupied only the centre of the photographs as captured. This was done to avoid possible vignetting by the lens affecting the luminosity of the corners. Caveats: illumination was not as even as it should have been.

This LED type seems usable for photography with a converted camera as a white light plus NIR radiation source, so that VIS and NIR images can be taken by only swapping filters. It should be also useful for focusing when using a VIS + NIR flash or a NIR flash.