Flight-worthy model of Euclid's dichroic filter
In order to fulfil its role, both of its surfaces are coated with more than 180 thin layers of di-electric materials. High uniformity of these coatings across the 117 mm diameter plate was required.
The dichroic was tested by Optics Balzers Jena GmbH, the coating manufacturer, for the spectral reflectance and transmittance – how much of the incoming light is reflected and how much is transmitted per wavelength.
The intense blue hue is a result of looking at the dichroic in transmission. A white light source is placed above the dichroic, and what we see is the portion of light, reflected by the table, that is transmitted through the glass to be directed towards the NISP instrument: this consists of blue light and of near-infrared light, the latter being invisible to human eyes. This is a design choice to minimise the amount of blue light that would be reflected towards the VIS instrument. The dichroic would appear in yellow-green hues when looked at in the direction of the reflected light, which is what the VIS instrument receives.