Just wondering how people are verifying accuracy to ISO 3664-2009 not only on your own viewing booths; but also on "clients" viewing booths? For years we were ordering bulbs through our graphics supplier. These bulbs do adhere to ISO 3664-2009; but we started seeing a rather large discrepancy between our two large viewing booths; especially with optically brightened papers. Booths were clean, no ambient light problems. It is important to note that the bulbs were not purchased through Home Depot type stores, but through a reputable Graphics supplier. We talked with the booth manufacturer who mentioned that bulbs could still meet the ISO 3664-2009 requirements while coming from different manufacturers (ex: Sylvania, Philips). Needless to say the viewing booth manufacturer said to order bulbs from themselves. We did that and it completely solved our in-house problem. I asked them about using the GATF Rhem indicators on customers booths and they implied that they would not work too well. If that is the case; how can we ensure that our customers (worldwide) are seeing what we are seeing. Is there a different "Rhem" indicator that might work, such as the one Ugra sells? Can we suggest that they purchase bulbs from the viewing booth manufacturer. It really is unfortunate that bulbs can adhere to the ISO specification but still create rather large visual differences. Any input would be appreciated. Thanks Mike Stewart