Design of the transmission / reflection device

The final concepts of the envelope and the screen are illustrated by the sequence of images given in Figure 3. In order to control precisely the illuminated sample area and thus minimize the parasitic reflections and blind zone, a quarter-circular frame supports a perforated sheet on which a motorized strip showing one circular aperture is unrolling, of diameter equal to the sample’s and facing the light source for any incident altitude angle 61. The sheet’s elliptic openings are of dimensions given by the apparent sample surface (accounting for inclination angle 0j) and are correspondingly positioned on the quarter circle arc.

(a) Strip hole over elliptic opening (b) Controlled illumination of sample

(c) Obstructing screen (d) Lifting of cover (e) Removal from path

Figure 3: Control of incident beam penetration and path through obstructing screen.

The projection screen concept relies on the removal of elliptic covers by a robotic mech­anism. The ellipses’ dimensions were again determined by the apparent sample area ac­counting for angle 0j, yet this time projected on a the screen surface, that is oblique to the sample plane with a tilt angle ©0 = 49.1°. The induced blind spot can thus be exactly reduced to the light beam’s area, which allows a minimal loss of information on the emerg­ing light distribution and negligible parasitic reflections around the sample area. Of course, a blind spot only appears for one of the six screen positions, except for normal incidence where the tip needs to be removed for all of them.

The optimal combination of altitude step Двг and sample diameter D is determined on one hand by the device’s geometry itself, and on the other hand by the minimal illuminated area required for advanced fenestration systems or coating materials characterizations; the mini­mal allowed sample diameter was thus found to be equal to 15 cm.

Once the concept’s applicability in practice was verified, the new components were designed and constructed.