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Monday, September 8, 2014

Lamp Life: Why does my instrument's lamp have to be changed so often?

Quartz Tungsten Halogen (QTH) Lamp
Very often customers question how often they should change their lamps.  We tell our customers to change QTH (quartz-tungsten-halogen) lamps in their S-4, Micro S-5, BNL-3, TB-1 and Micro TB-1C instruments every 3 months. Over the years we have noticed three things that start to happen to the lamps at the 3 month time frame.

1) The UV level of the lamp degrades as the lamp ages.  The UV level will start dropping faster at the 3 month period of time which will effect readings on any product with optical brighteners.

2) The instrument will start having minor stability issues. The repeatability and reproducibility is reduced due to the degradation in the lamp.

3) The spectral response from the lamp changes which will affect the response of the instrument to reflectance measurements at different wavelengths.  This may affect color values, brightness and/or opacity which are all measured at specific wavelengths.

All of these are due to degradation in the lamp over time.  By replacing the lamp on a regular basis and recalibrating the instrument, the customer should not see any large shifts in their readings.

There are two options available to users:

1) Make sure to change lamps in the instruments at least every 3 months.  The small cost of a QTH lamp is far less than the cost of a customer claim or return.

Xenon power pack & bulb
2) In cases where the instrument is a diffuse geometry (TB-1 or Micro TB-1C), use a Color Touch spectrophotometer instead of a colorimeter. The Color Touch uses a xenon lamp which is filtered to achieve the different UV levels necessary for optical measurements. The xenon lamp will last for 5-20 years (depending on usage).

PS - Don't be fooled in to thinking that xenon lamps need to be changed on an annual basis. They have a lot of UV energy. Even over years of use the UV level of a xenon lamp will be high enough to prevent the issues described above.  If your instrument manufacturer suggests that the xenon lamp be changed in a period less than 5 years, ask why and ask to see data showing the degradation in the lamp.

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