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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 only 3% for the Cary internal 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 3: Some of the wide-angle scatter is lost when there is a space between the

sample and the sphere wall

Sample recess

Theory assumes that the sample is placed coincident with the inside of the sphere

wall, however the sample is placed against the outside of the sphere wall. The

porthole edges have a finite thickness, and some part of the beam reflected at wide

angles may be intercepted by the sphere wall.

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