HYPERSPECTRAL IMAGING
to see if any gels leaked liquid onto the outside of the cap. Te other issue with halogen bulbs is lifetime;
a halogen bulb lasts only about 1,000 hours compared to LED light sources that have 50,000 hours or more lifetime. Halogen bulbs also quickly lose intensity. In addition, there is now a lot more pressure
to come up with an alternative to halogen, Gardiner noted, because, as of September 2018, the sale of halogen bulbs in the EU is banned as part of the final phase of new EU energy regulations. Tere is also no flexibility in the spectral
profile of a halogen light source; it is broad and there are no gaps in it, but it’s not particularly flat, according to Simon Stanley, director of technology at LED light manufacturer ProPhotonix. ‘Output [of halogen] is high in the IR region but almost non-existent in the UV and blue,’ he explained. ‘With an LED light, by contrast, we can replicate and adjust certain portions of the halogen spectrum, so we can make it flat, for example, or make it biased to the UV or IR side, or whatever the application might require.’ ProPhotonix offers LED products for
both multispectral – a set number of distinct wavelength bands, rather than a continuous range – and hyperspectral applications. Its Cobra MultiSpec line light has 12 separate colour channels, which can also be configured to provide a complete spectrum over a wavelength range. Stanley said that one of the things the
company is sometimes asked to do is bolster the LED spectrum in particular areas, such as when a camera sensor doesn’t have equal sensitivity across the spectral range. ‘For example, if the camera isn’t particularly sensitive in the infrared, we might bolster the infrared part of the spectrum, while still providing a full spectrum into the visible,’ he explained. Te other downside to halogen bulbs is that
they are typically not optics-friendly, Gardiner noted. Halogen spreads its light out over a large field of view, making it difficult to focus that light into a line, which would be beneficial for line scan cameras, the most common hyperspectral camera. One of the advantages that Specim cites for its FX cameras, is that they only require narrow line illumination, whereas a much larger area needs to be illuminated for filter-based cameras. ‘Halogen lights have a lot of wasted energy and photons that could have been directed at the object,’ Gardiner continued. LEDs, however, can use focusing rods or diffusers to give uniform focused light.
The coating thickness on Delta Optical Thin Film’s continuously variable bandpass filters change along one direction of the filter
‘Hyperspectral imaging requires a lot of
light,’ Gardiner stated. ‘A spectrometer using a combination of optics to split the light into 400 or more different spectral bands needs quite a lot of light to image all those different bands. Te light needs to be high power and also emit a broad spectral range, which isn’t necessarily natural for LED technology.’ Metaphase worked with its LED partners to
create custom LEDs for certain hyperspectral projects. ‘A natural LED, in terms of spectral band, tends to be very narrow, which means
We can offer our sensors, similar in performance to InGaAs, at a price that is 20 to 30 per cent less than InGaAs
you need a lot of LEDs to cover a broad spectrum,’ Gardiner explained. ‘We take these narrowband LEDs and use custom combinations of phosphors to convert some of the photons emitted to a different wavelength. Tis phosphor correction expands the spectrum produced.’ Tis is done in combination with custom
chips to make a single point source LED much broader in spectral coverage. Metaphase’s visible NIR light, for example, uses five LEDs: a broadband blue LED emitting from 400nm to 500nm; a broad white LED for up to around
16 Imaging and Machine Vision Europe • December 2018/January 2019
700nm; and then three different LEDs to cover the 700nm to 1,000nm range. A typical blue LED might only cover around 15nm of the spectrum, so the firm uses phosphor corrections to expand the number to a half width, half max of 100nm – the phosphors shiſt the photons from the LED to cover a broader spectrum. Photons are lost in phosphor correction, but
Metaphase is using custom high-intensity chips that produce a lot of photons, so there are more photons to convert in the first place. Metaphase designs optics to take the
wavelengths of those different LEDs and blend them together into homogeneous illumination. It then uses a focusing rod to focus the light into a tight, narrow, high-intensity beam. Te company continues to improve on its
phosphor and chip combination to give a broader spectral range and produce a flatter spectral curve. ‘A flat spectral curve generally does help, because the user has to do less pre-calibration of the camera, such as spectral balancing or white balance correction,’ he said. Because hyperspectral LEDs tend to be
custom-made, they are more expensive than standard white LEDs for machine vision, Gardiner said. Te firm’s SWIR hyperspectral light – which is its most popular hyperspectral light – is made from InGaAs chips, which are more expensive to manufacture. ‘For some applications, that pricing will be a hurdle, but usually the other components, such as the camera and the soſtware, will be expensive in itself, so it tends to be an expensive system
@imveurope
www.imveurope.com
Delta Optical Thin Film
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