uv and measurements.
uv and measurements.
- Subject: uv and measurements.
- From: email@hidden
- Date: Wed, 23 Dec 2009 14:57:20 +0000 (UTC)
The whole topic of UV in measurements is currently being examined with care from many different standards bodies. There is a new form of standardization within ISO that basically describes four standard measurement conditions:
1: M0 legacy light source unspecified
2: M1 D50 illuminant or illuminant condition where the ratio of UV to visible matches D50
3: M2 UV cut
4: M3 Polarized.
The reason that M1 is being advocated by organizations such as TC130 is that it finally specifies the amount of UV that is applied to object being measured. One key component of M1 is the ratiometric consideration of UV to Visible with respect to D50. This specification assumes that the fluoresence component is independent of absolute amplitude. I built a lab instrument to investigate this and I found that reciprocity generally holds true, so the specification does make sense.
The average user has absolutely no control of the UV in the light booth, the instrument, or the nature of the Optical Brighteners in the paper. I , personally, would never recommend using an M0 device for general profile generation, but many very good profiles have been generated using such instruments. The interaction of the source and the paper is totally unspecified. Many engineers will say that a tungsten bulb will put out enough UV to excite the OBA in the paper. This is completely true, but the amount of UV in an M0 instrument is very variable and it is a very strong function of the regulation of the lamp. MO devices have two unspecified sources of potential error: the UV content of the source and the amount of OBA in the paper.
Why use UV cut (M2)? In theory UV cut should give the media independent readings and provide better correspondence between instruments. M2 is best suited for applications that involve quality control and measurements of ink on paper without regard to the substrate, per se. It is important to note that if one is converting a spectral measurement to a Status Density measurement, the legacy M0 condition should be used. The status density specifications generally demanded a tungsten based illuminant.
To the best of my knowledge, no one actually builds an M1 instrument. It is not trivial because the characterization of the UV is not easily accomplished. A filter that would map a typical tungsten source to d50 that maintains the ratio demanded by the M1 specification would reduce the total output of the source by a factor of roughly 375. A traditional white LED has virtually no UV component, so an led based instrument would require compound illumination.
Measurements of many popular light booths show that there has been virtually no compliance with the UV component of d50. This fact supports use of M0 or M2, because there is no control of UV in the booths, why would M1 do any better? The old MacBeth SP3 booths actually do quite will with filtered tungsten and custom filters, but they require large amounts of power to achieve that goal (remember that factor of 375X loss of total energy).
So what would I recommend:
If you are making a measurement that is intended to represent the visual process, the M1 condition should be used, but M0 has shown great utility albeit with a high degree of inter- instrument variablity. Keep in mind that the variability in light booth output swamps any of the differences you would see between M0 instruments. For this reason, one should expect a lot of push back from manufacturers against M1 until manufactures of light booths take the UV component seriously as well.
If you are making a measurement that is to be transmitted to another facility for confirmation, the M2 (UV cut) should be used because it minimizes the differences in UV content in illumination between instruments and it makes no assumption about viewing conditions. One could make the argument that because the UV content in most viewing environments is so out of control, making a profile with UV cut should be OK because the effect of UV is totally out of the profile maker's control.
I believe that we (X-rite) offer a visual solution which allows inclusion of the UV component in a light booth to be applied to profile. This process links the profile to the viewing booth which is important in some situations, but keep in mind that this making a profile both instrument dependent and light booth dependent.
There is no easy answer to the UV issues because they are so totally out of control in the market place. There are also different requirements, some require UV and some need to avoid the effect of UV. With this in mind, it should come as no surprise that the consumer is confused.
Regards,
Tom Lianza
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