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Factors affecting accuracy or precision

Below are the major factors that may affect the accuracy of measurements when using the

DRA.

Aperture area/total surface area ratio

Some of the reflected light escapes through the ports. This has the effect of reducing the

signal to noise ratio, and thus the precision of the measurement.

The Commission Internationale de l’ Eclairage (CIE) recommendation is a ratio of <10%.

This figure is < 10% for the Cary External DRA.

Coating non-uniformity, ageing, or contamination

This accessory has been coated with PTFE via a unique process that ensures both a

uniform coating on the inside of the sphere and the correct powder density. PTFE is

durable, and does not yellow. The reflectivity of the PTFE is above 96% between 200–

2500 nm, and greater than 99% between 350–1800 nm. The sphere will maintain its

reflectivity indefinitely if not subjected to smoke or other contaminants. Contact with

plastic materials may also contaminate the coating.

Incorrect sample placement

Theory assumes that the sample placement will coincide with the inside of the sphere

wall. The sample is normally placed at a port on the outside of the sphere wall. Spacing

between the sample and the sphere wall can lead to large errors due to loss of reflected

light.

Figure 5: Some of the wide-angle scatter is lost when there is a space between the

sample and the sphere wall

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