FAQs
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INTEGRATING SPHERES
1.) How often should I recalibrate my sphere or sphere system?
Answer:
We recommend a period of a minimum of one year between optical calibrations
for systems. Sphere stability is primarily based on cleanliness (dirt, dust,
finger oil) – the nicer you are to your sphere – the longer it
will last.
2.) How often should I recoat my sphere?
Answer:
Coating longevity is almost infinite – when it is not used. Usage is
the prime factor in determining when a recoating is necessary. If the sphere
has chips, cracks, excessive dust, a yellowish appearance, fingerprints, or
burn spots it should be recoated as soon as possible to ensure continued throughput
accuracy.
3.) How does the size of sphere affect the throughput of
the sphere?
Answer:
The throughput of the sphere is related to the ratio of the area of the exit
port to the area of the sphere, and the surface area of the sphere is proportional
to the square of the diameter. Therefore if the size of the exit port remains
constant, then doubling the diameter of the sphere will decrease the throughput
by a factor of 4.
4.) If the reflectance of the sphere coating is so flat
why is my throughput so dependant on wavelength?
Answer:
The Zenith® and Optowhite sphere materials have exceptionally high and
very stable reflectance. However, due to the nature of the sphere’s multiple
internal reflections, even slight variances in reflectance vs. wavelength are
augmented. For instance, if there are an average of 15 bounces for a ray of
light within the sphere, the 99% reflectance is applied as (99%)^15 = 86% compare
to 98% reflectance is applied as (98%)^15 = 74%. What this is saying is that
a 1% difference in reflectance results in a 12% difference in throughput. This
is why small variations in spectral reflectance cause large variations in spectral
throughput.
5. What is the maximum number of ports I can put in a sphere?
Answer:
As a general “rule of thumb”, the total combined surface area of
the sphere ports should be kept to <5% of the total sphere surface area.
LUMINANCE/RADIANCE STANDARDS
1.) What is the largest port that the sphere will allow
before uniformity is degraded?
Answer:
If the ratio of the port diameter to the sphere diameter is 1:3 you can expect
a uniformity of +/- 1%, if that ratio is 1:4 you can expect a uniformity of
+/- 0.5%. In general, the higher the reflectance of the sphere coating the
better the uniformity, therefore a Zenith® (PTFE) sphere will be more uniform
than a Optowhite (BaSO4) with identical port to sphere ratios.
LAMP MEASUREMENT
1.) What is an auxiliary correction procedure and why is
it necessary for accurate lamp measurements?
Answer:
When a lamp is placed into an integrating sphere, it absorbs light even while
it is radiating in the Sphere. This absorption is slightly different for every
lamp and is certainly different for a lamp with a black base (very absorptive)
versus a white-based lamp. An auxiliary lamp corrects for the difference in
absorption characteristics of the reference lamp versus the test lamp.
LASER POWER MEASUREMENT
1.) How do I determine the throughput of my integrating
sphere system for a given input power?
Answer:
Use the following equation to calculate the throughput
of your sphere.
Whereis
the ratio of energy falling on or exiting through the area
Aout to the input energy, is
the average reflectance (including ports) of the sphere wall,
the FOV is the field of view, measured in sterradians of
the area Aout .
The following equations may be helpful.
Aout = π r2 and Asphere
= 4 π r2 where r is the radius of the output
port or the sphere.
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