Hyperspectral imaging needs relevant illumination. This may cause heating on the samples. This article illustrates how heat is transferred to the samples while imaging, especially during artwork inspection.
Several illumination sources are available for hyperspectral imaging. Some of them are hot and may heat the samples. You need to address careful attention so that one does not damage the samples.
An application where temperature needs monitoring is the inspection of paintings. This article is written within this context. However, conclusions are still valid and transferable for other applications.
VNIR – SWIR illumination
Several light sources are available to cover this spectral range: sun, LEDs, supercontinuum lasers, and halogens. Since artworks are often placed in conservation laboratories, artificial lights are needed.
For color inspection, you only need the visible part of the spectrum. Hence LEDs are relevant, and their absence of heat makes their use even more relevant. However, for art material inspection, an IR light is needed. Those penetrate deeper into the painting and may reveal valuable under-drawing. Thus halogens are primarily used because they can cover the entire spectral range (400 – 2500 nm) at a low expense.
The issue with halogen illumination is the heat. Those may increase the paint surface, especially on the darkest areas. Black painting includes carbon (probably), and carbon absorbs through the wavelength range between 400-2500nm.
This can be reduced simply by adding a fan to move air close to the surface. A simple test was made with a very dark absorbing oil painting to show this. The artwork was illuminated with 2×45° line illuminators, and the total power was 200W (this is relatively high). The distance to the surface was 50cm. The following image shows temperature increase within the first 5 minutes with and without the fan. The fan was installed parallel to the surface.
Figure 1: Temperature increase on painting surface with and without a fan.
However, the user needs to keep in mind that those measurements were static. Specim cameras are line scan devices and need movement to measure an entire painting, limiting the heat load.
MWIR- LWIR thermal illumination
Still, within the context of painting inspection, the heat produced by Specim MWIR – LWIR radiators has been tested on samples. Here we highlight the influence of the scanning speed.
Figure 2: Temperature increase on painting surface at two different scanning speeds.
The blue curve represented the temperature rise when the scanning tray moved at 100 mm/s, whereas the red curve corresponds to a scanning speed of 200 mm/s. For each curve, two temperature raises appear as they relate to the scanning and retracing of the scanner (meaning that twice the sample was passed under the illumination). The surface temperature of the sample rose by +4 and +7.5 degrees at respectively 200 and 100 mm/s.
Once again, using a fan would limit the rise of the sample temperature.