The amount of optical overfill on the critical area of the window aperture cannot be measured directly. For systems with uniform illumination on the array the amount is determined using the total measured incident optical power on the DMD, and the ratio of the total optical power on the DMD that is on the defined critical area. The optical model is used to determine the percent of optical power on the window aperture critical area and estimate the size of the area.

• Q_AP-ILL = [Q_INCIDENT × OP_{AP_ILL_RATIO}] ÷ A_{AP_ILL} (W/cm²)

where:

• Q_AP-ILL = window aperture illumination overfill (W/cm²)
• Q_INCIDENT = total incident optical power on the DMD (Watts) (measured)
• OP_{AP_ILL_RATIO} = ratio of the optical power on the critical area of the window aperture to the total optical power on the DMD (optical model)
• A_AP-ILL = size of the window aperture critical area (cm²) (datasheet)
• OP_{CA_RATIO} = percent of the window aperture critical area with incident optical power (%) (optical model)

Sample calculation:

See the figure for the length of the critical aperture.