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The intensity of the solar radiation on the front of the objective is very low, 98% of the energy goes straight through the lens and there is very little, if any, heat build-up on the front.
All the heat and most of the light also goes through the Herschel wedge, hence the heat trap/ deflector/ diffuser on the rear of the wedge. About 4% is reflected towards the eyepiece/ CCD.
An ND3.8, as a minimum, MUST be used between the wedge and the eyepiece/ CCD. I also add a variable ND filter and the Continuum to get the most comfortable view.
In normal use there is NO heat build-up at the eyepiece/ CCD - it's very safe.
I initially built my own Herschel wedge using a 60mm, 30 degree prism from Surplus Shed - worked 100% and gave much better results than either of my objective filters (JMB, Baader Solar film).
The "Cool" ceramic heat sink on the Baader design is very effective, and there is no hot exit beam. All the other designs, Lunt, Intes etc dump the excess energy out the side of the wedge housing.
HTH
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