Photospectrometer - newbie question

[mike9009]

Hi

I was wondering if there are any experts on PH who know about photospectrometers?

I am looking at the repeatability and reproducibility of photospectrometers across brands/ regions/ machines. If I 'read' a colour chip on one machine and obtain some 'Lab' numbers, how repeatable would the Lab numbers be on another machine using the same illumination and other settings. (Assume the machines have been calibrated)

Reason : I have a customer who is using brand X handheld photospectrometer who is claiming the delta E between two readings is 5.92. We currently match using visual inspection and are in the process of purchasing a similar machine (not identical). We consider the colours to be matched. I 'suspect' they are using 'Lab' numbers taken 8 years ago and comparing to a recently manufactured colour chip. The colour in question is a dark grey, if relevant?

A web search and talking to manufacturers of photospectrometers has been a little fruitless, but we are having some training once the machine has arrived.

I want to set up a protocol for measurement which will provide a consistent approach from all parties. The materials we are matching are thin polycarbonate plastics, so they do have a translucency which I suspect is causing issues too.....

Thanks in advance


Mike

[Moonhawk]

I have some experience with spectrophotometers and densitometers

I think what you'll probably need is a set of traceable standards that each lab can use.

The reading from this standard can then provide a common baseline against which other measurements can be measured against. Your standard would essentially baseline correct your environment/machine and you would quote a reading relative to this standard - rather than an absolute reading.

Think of it a bit like how a photographer uses a grey card to correct for white balance and exposure under different lighting conditions.

Edited by Moonhawk on Wednesday 7th December 21:00

[mike9009]

Moonhawk said:

I have some experience with spectrophotometers and densitometers

I think what you'll probably need is a set of traceable standards that each lab can use.

The reading from this standard can then provide a common baseline against which other measurements can be measured against. Your standard would essentially baseline correct your environment/machine and you would quote a reading relative to this standard - rather than an absolute reading.

Think of it a bit like how a photographer uses a grey card to correct for white balance and exposure under different lighting conditions.

Edited by Moonhawk on Wednesday 7th December 21:00

Thank you, this is exactly what I suspected. Doing comparisons of delta e against a known, agreed standard rather than against a set of numbers makes sense. Is this standard industrial practice or a potential solution to my individual circumstances? Is there an ISO standard or any guidelines for setting this up between companies?

Thanks

Mike

[Monty Python]

All our spectrophotometers are calibrated against a white tile and black trap. The white tile is specific to a particular instrument so you can't swap them around.

[Moonhawk]

Monty Python said:

All our spectrophotometers are calibrated against a white tile and black trap. The white tile is specific to a particular instrument so you can't swap them around.

That's ok as long as the specific white tile is itself calibrated against a standard.

[Moonhawk]

mike9009 said:

Thank you, this is exactly what I suspected. Doing comparisons of delta e against a known, agreed standard rather than against a set of numbers makes sense. Is this standard industrial practice or a potential solution to my individual circumstances? Is there an ISO standard or any guidelines for setting this up between companies?

Thanks

Mike

It's common practice in any analytical lab. You can get standards for many types of analytical test.

The manufacturer of the instruments you are using to take these measurements should be able to advise - and i'd be very surprised if they couldn't provide reference standards (at a cost of course).

The manufacturer of the instrument below provide reference standards for their instrument.

http://www.stellarnet.us/systems/color-measurement...

[Monty Python]

Moonhawk said:

That's ok as long as the specific white tile is itself calibrated against a standard.

That's done at the factory using standards provided by the NPL - the data is then put into the software so that when the white tile is measured during calibration, any significant deviation can be spotted and minor deviations adjusted for.

We have around 30 of these instruments from various manufacturers and they all work the same way.

[Moonhawk]

Monty Python said:

That's done at the factory using standards provided by the NPL - the data is then put into the software so that when the white tile is measured during calibration, any significant deviation can be spotted and minor deviations adjusted for.

We have around 30 of these instruments from various manufacturers and they all work the same way.

Yep - it is important however to have your 'internal' standard re-calibrated/verified every now and again because they can drift.

Our check weights at work for example are verified every 12 months.