Diffuser's Calibration Report

Drawing with dimensions of diffuser BAT 5
The diffuser’s distance to fibre distance is matched to the fibre’s N.A. There is a slight slope in the diffuser’s surface to drain the water through a drainage hole. An 'o'-ring gives additional protection against rain.
The diffuser can be made in almost any size.

DCE,Q?

DCE,Q?

Setup
The following setup was used for testing the cosine response and transmission of the diffuser.
In this setup the lamp distance is as large as possible. The reason for this is that the lamp should resemble a point light source. If the lamp is placed at a close distance this would lead to large errors especially at bigger zenith angles. The diam eter of the windings inside the lamp are about 5 mm. A high accuracy power supply is used to keep the lamp current constant (Optronics OL 83A). The voltage across the lamp filament is monitored directly.

Quality factors: Relative cosine error, Q and DCE
Relative cosine error
The quality of the diffuser is defined with the following quality factors.
The "relative cosine error" is defined by:
Q correction factor
Q is a correction factor for isotropic radiation:
This factor 'Q' is greater than 1 if the diffuser overestimates and less than 1 if the diffuser underestimates.
Diffuse Cosine Error (DCE)
The "Diffuse Cosine Error" shows the actual quality of the cosine response response.
Physical meaning of the DCE: If the diffuser has a perfect cosine response this factor will be zero.

PTFE is an excellent diffuser material for instruments exposed to the elements: It has a low coefficient of thermal expansion (0.000122mm per degree Celcius) It has a low moisture absorption (< 0.01%) It is stable against damage by UV radiation The following graph shows the measured temperature-sensitivity of teflon for the temperature range 19 degrees Celcius and 45 degrees Celcius.
Info, questions and remarks, contact:
Richard McKenzie [ [email protected] ]

For this experiment the diffuser is placed at a position which is 20 mm greater than the normal fibre position. In between diffuser and fibre is a teflon "light tunnel". The cosine response is shown below:
The graph shows that the cosine response does not change dramatically, altough it improves a little.
This suggest that the diffusers do not necessarily have to be used in combination with a fibre.

Sometimes it is neccessary to disconnect fibre and diffuser. Reconnection can cause a change in the diffuser-fibre distance, which could lead to errors in previous calibrations. Thermal expansion can also be a reason for a changing diffuser-fibre distance .
The following graph shows the change in photomultiplier signal due to a changing fibre distance.
When the fibre is relocated with care relocation errors are around 0.1mm.

The position of the diffuser's entrance aperture needs to be known for accurate calibration. Because the diffuser has a hemispherical shape the effective entrance aperture is not exactly located at the front surface.
Experiments with the lamp at 1.5 m and 0.5 m show that according to the inverse square law the location of the effective entrance aperture is 1.4 mm (±1mm) behind the front surface of the diffuser.
Info, questions and remarks, contact:
Richard McKenzie [ [email protected] ]

While in use it is best to clean the diffuser on a daily base with a piece of dry cotton. This to wipe of any dirt, dust, snow etc. If the diffuser for some reason gets really dirty it is recommendable to clean it with water and soap or alcohol. Dust inside the diffuser can be removed with a cotton bud.
Info, questions and remarks, contact:
Richard McKenzie [ [email protected] ]