Hyperspectral cameras measure light, which is emitted, transmitted, or reflected by objects. In the cases of reflection and transmission, the incident light plays a significant role in the relevance of the results. Since hyperspectral cameras measure continuous spectra, it is fundamental that the light source has a continuous spectrum too. Several light sources are suitable and available with such property.
Halogens are probably the most common light source for hyperspectral imaging and the cheapest. Typically, halogens emission follows the Planks curves of black bodies between 3000 and 4500 K (see Fig.1).
Figure 1: Typical emission curves of halogen bulbs.
Notice that warmer halogens contain more blue signals than colder ones but, on the other hand, have less IR. Also, warmer halogens are less balanced than colder ones. You can use a spectral flattening filter to even this.
Continuous spectra over 400 – 2500 nm
Good IR emission
Requires optical mechanics to be focused (like elliptical reflectors)
Has poor blue emission and high intensity at ca. 1000 nm, which is not optimal for color measurements.
Since the color measurement is required in many applications, some halogens have been modified with filters. These halogen lamps also have a reflector that does not reflect infrared radiation forward but lets it pass backward, reducing the infrared heat load to the surface. These lamps cannot be used in NIR or SWIR measurements.
2. Line light based on cold halogens
To obtain a collimated line light from a halogen source, it is also possible to use an optical guide (fiber) with a cylindrical lens.
Collimated line light
Continuous spectrum over 400 – 800 nm
No IR signal
Poor blue emission
LEDs are quickly developing since the middle of the 2010s, and several are now available for hyperspectral imaging. However, all are not suitable, and only so-called white ones are suitable. Several manufacturers propose such LEDs.
Can be tuned
Collimated line light
More expensive than halogens
Not very bright
Uneven signal with sudden drops
Poorly available above 900 nm
For color measurements, those are rather good since the signal in the blue end is good.
4. Supercontinuum laser
This is a relatively new technology for hyperspectral illumination. Several options with different spectral ranges are available.
Bright collimated line
High peak at 1064 nm
Spectrum at the middle of the line is different from the edges
The signal dropped significantly after 2350 nm
Class 4 lasers, which require safety instructions for integration and usage.
Those are typically used for thermal imaging. They are emitters like halogen bulbs are, but at lower temperatures, their emission peak is between 3 and 12 um. Specim produces one whose temperature is at 900K.
Figure 2: Specim thermal illumination and the related black body emission curve.
Specim MWIR and LWIR cameras cover a spectral range of 2.7 – 5.3 and 8 – 12 um. Those ranges are located after the emission maximum of the illumination to be as flat as possible on the corresponding cameras wavelengths. A warmer illumination would have a steeper slope over the thermal spectral range, whereas a colder one would have too low an intensity. There are other potential suppliers of thermal illumination sources like Elstein and Toshiba.
Continuous spectrum over the thermal spectral range
Covers the full MWIR – LWIR spectral range
Need reflectors to be collimated (as SPECIM one)
Needs to be handled with care (hot element)
6. Sun light
The solar illumination is used in many applications: precision agriculture, environment assessment, mining industry, and all related to remote sensing applications. By itself, the sun provides a continuous spectrum illumination that a black body would emit at about 5800 K. However, the atmosphere surrounding the Earth contains gazes with absorption features, shaping the illumination reaching the surface of our planet.
Figure 3: Sun emission and atmospheric absorption.
As shown in Fig.3, solar illumination is very relevant between 400 and 1000 nm, but above this, atmospheric models need to be involved in correcting spectra when possible.
A relatively continuous spectrum over 400 – 1000 nm
Available only during the day
Clouds affect the IR emission
Atmospheric absorption peaks
Unlike the other illumination sources, for fluorescence measurements, light is needed to trigger a specific type of emission from the samples. This is not used for reflectance or transmission. Therefore, a continuous spectrum over the entire spectral range is not required. Typical light sources used for fluorescence cover a relatively narrow spectral range in the UV or are laser-based.
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