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Figure 4: Some of the wide-angle reflection is intercepted by the sphere wall

The edges of the reflectance port are feathered to reduce this error. A protruding

PTFE reference plate is provided with the accessory to more accurately represent the

perfect diffuser.

Reference beam attenuation

Reference beam attenuation is most useful when the accessory or sample in the

sample beam attenuates the light beam considerably. In such situations, attenuation of

the reference beam will increase noise and considerably increase the dynamic range of

the instrument, as the detector is not then 'seeing' two dramatically different signals.

Clips are provided at the reference beam window for this purpose. Metallic mesh

screens are recommended if reference beam attenuation is required.

Stray light

If the irradiating beam overfills the reflectance port, a proportion of sphere wall

reflectance is mixed with that of the sample. This gives a high reading if this is not

consistent between the baseline and the sample measurement.

Gloss trap error

Gloss trap error is produced when the gloss trap is unable to completely absorb the

specular component. In the measurement of diffuse-only reflectance, a 'gloss trap' is

often used to absorb the specular component. Gloss traps are typically glossy black

pyramidal light traps, matt black-coated cavities, or razor blade Fresnel light traps.

However, the reflectance of many samples has broadened the specular peaks which

cannot be excluded by standard sized light traps. The Cary DRA eliminates this

source of error entirely by reflecting the specular component out of the transmission

port.

Differences between the standard and sample

It is important that the reference material be of a similar reflectivity and have similar

properties to the sample. Otherwise large errors can be introduced, if for example a

poorly reflecting material is measured relative to a highly reflective PTFE reference

plate. The table below lists the appropriate reference materials that are recommended

for use.

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