Measurements at high absorbance are among the most demanding tasks for a UV-visible spectrophotometer, with factors such as instrument linearity, stray light, and detector noise all conspiring to introduce potential errors. This situation is exacerbated when very small samples are being measured. An investigation at this manufacturer of spectrophotometer cells (cuvettes) compared results obtained using conventional clear-walled micro and semi-micro cells with those using cells with self-masking solid black walls. Measurements in their UKAS accredited (ISO 17025) calibration laboratory revealed that light leakage through the cell walls of unmasked cells could introduce errors of over 40% at high absorbance levels (3A). Significant errors were detected at absorbance levels well below 1A. The company's self-masking micro and semi-micro cells area available with a variety of window materials and path lengths from 5 to 50 mm, and are assembled using a fully fused method of construction pioneered by the company. Experimental results obtained during the investigation can be supplied upon request.