where Isun and Imeas are the normalized intensity of the
sun and the tungsten lamp with or without filters,
respectively. Small periodic oscillations in the recorded
signals were filtered with a low pass Butterworth filter using
LabView [15]. The spatial uniformity was determined by
measuring the light intensity over an irradiated area of 250 x
250 mm2 using an affordable solar power meter (Amprobe
Solar-100, $119 US). The sensor of the solar power meter
was positioned at different equally spaced locations within
the irradiated area and using a shadow mask of 25 mm2
aperture, where the irradiance at these locations was
recorded. Spatial non-uniformity (SNU) was calculated as
follows: