24. January 2018

Progress with the illumination of HSI cameras

The use of hyperspectral cameras for different applications in the medical and life science field requires an appropriate source of illumination with the illumination requirements being determined by the evaluated spectral range. The stability of the illumination over time and the distribution of the intensity in the area of interest of the spectral range are crucial for the chemometrical evaluation of the 3D data cubes with constant algorithms.

Frequently, illumination units which are based on thermal halogen lamps are used because for absorption spectroscopy, a broadband stable illumination is required. These illumination units have become well-established and dispose of a multitude of positive characteristics for most applications. Beside their positive characteristics, halogen lamps often have limitations which prevent the use of HSI systems in particular applications.

That is why Diaspective Vision for quite some time deals with alternative illumination possibilities for HSI systems. During the past view months, substantial progress could be achieved: the first preliminary surveys with light sources based on Xenon lamps could be conducted. Xenon lamps are rated critically for the use in HSI applications because the emission spectrum of the lamps combines continued radiation with line emission which is of disadvantage in hyperspectral application.
The following illustration shows a test occlusion study where a Xenon light source is used as illumination unit.

Okklusionssequenz aufgenommen mit einer TIVITA® Tissue Kamera kombiniert mit einer 300 W Xenonlichtquelle
Occlusion sequence recorded with a TIVITA® combined with a 300 W Xenon light source

It is apparent that by slightly adapting the software, the light sources become suitable as illumination units for the use in TIVITA® cameras.
Another highly interesting option could be an LED based illumination. In the meantime, white light LEDs are available for private use at a good price. They become more and more popular as they – through their limitation on visible light – generate much less thermal radiation than lamps.
But an advantage on one hand can become a disadvantage on the other one: As these LEDs do not emit near-infrared light, they are not suitable for the use as illumination for HSI systems in the VIS-NIR range.
Approaches with a combination of a multitude of single-coloured (monochrome) LEDs failed in the past, because the spectral emission of these LEDs depends on temperature and therefore the illumination units become astable in consequence of fluctuations in temperature and power.

For these reasons, Diaspective Vision for some time follows the approach of broadband LEDs based on broadband emitting luminescent substances. In the meantime, the technology was developed thus far that camera recordings with LED based illumination units in the spectral range of 500 to 900 nm can be made with TIVITA® Tissue. Below, one of the initial recording series of an occlusion study with an in-house developed prototype of such broadband LED illumination unit is displayed.

Okklusionssequenz aufgenommen mit einer TIVITA® Tissue Kamera kombiniert mit einer breitbandigen LED-Lichtquelle
Occlusion sequence recorded with a TIVITA® combined with a broadband LED-light source

The following illustration shows the related spectral analysis pattern of an occluded recording.

Spektrale Analyse einer Okklusionsaufnahme mit einer TIVITA® Tissue Kamera kombiniert mit einer breitbandigen LED Lichtquelle. Die aufgenommenen Spektren zeigen sehr deutlich die erwarteten und bekannten Eigenschaften von oxygeniertem (Gelb und Blau) und deoxygeniertem Hämoglobin (Rot und Grün)
Spectral analysis of an occlusion recording with a TIVITA® Tissue Camera combined with a broadband LED light source. The recorded spectra explicitly show the expected and well-known characteristics of oxygenated (yellow and blue) und deoxygenated haemoglobin (red and green)

Development is not finished yet and further optimization steps are planned, but with these recordings the use of LED based illumination sources in combination with HSI systems for applications in the field of medicine and life sciences is foreseeable.
The illumination sources will offer big advantages for the perfusion evaluation in practice.
Thanks to the quick circuit times of the LEDs, the adjustment of extraneous light can be accomplished in the measurements and thus the recordings become less affected by the effects of extraneous light.
Besides, the heat introduction onto the examined tissue or object is significantly reduced once more, which is of advantage in many applications. For example with TIVITA® Tissue, during the recording of image sequences no change in perfusion will occur through warming of the skin.

Do you have any questions concerning broadband LED illumination for your spectral application? Do not hesitate to contact us! We would be pleased to assist you with our products and our extensive knowledge.