Collimator light illuminance 1

[DxDave]

Good day,
My question concerns the light field for a Bennett x-ray unit with model number HFQ-10000P. The collimator model is a Bennett D050M. Performing a radiation safety survey on the unit revealed the light field illuminance to be less than 160 lux measured at an SID of 100cm. The service engineer indicated that there was a circuit that produced a low illuminance for several seconds and then would come up to full illuminance level after a given time. He said I was not leaving my photometer in the light field long enough to measure the true illuminance level. Seems like an unnecessary feature. Is this the way Bennet intends for the light field to work? The unit is an hour's drive away so I have not been back to retest, and obviously its a minor issue, but I am curious.

Thanks for your help.

David Englehart

 

[Electromotive]

Hi! DxDave,
Sorry for such a late reaction. I joined eng-tips recently.
I hope you may find some help from my answer.

Switching-ON current of the collimator lamp is usually reduced by using a power resistor in series with the lamp for less than one second after switching ON.This is done to ensure longer life of switching circuit(relay, switch, lamp etc). In this period the lamp is not fully lit. When the full voltages are put across the lamp by removing the resistor the operator is not able to note the transition due to short switch-over time.
The current may also be reduced by applying low voltages in the begining and then shifting to proper operating voltages to reduce the starting current(lamp resistance is low in the begining due to less temperature), but this also should require not more than a second.
I am not familiar with the Bennet collimator type neither have I come across any collimator working in this fashion. GE, Siemens, Picker, Toshiba, Shimadzu collimators don't exhibit this function of a few seconds delay for full power-illuminance.
There may be a problem in timer circuit of the collimator.
Regards
Electromotive

 

[DxDave]

Electromotive

Thank you very much for your reply. The information is very helpful

Dave