“Blem” and “Dark Spots.” What do they mean?
Dark Spots:
Dark spots are inherent to the manufacturing process. Could be a particle sandwiched in the intensifier, fiber optic, or various other circumstance. All tubes are allowed dark spots. This is why there is a prescribed dark spot allowance. Dark spots do not affect intensifier performance and in no way indicate a “good” or “bad” tube. Factory dark spots are permanent as they’re a byproduct of construction. They cannot be cleaned or black boxed away.
Blem:
A blem is a user induced burn, shade, streak, splotch, debris, or other artifact. Some blems may be reversible depending on the severity. If you know it’s a burn you can try black boxing the device for several batteries worth. If debris, they can usually be resolved by servicing the device. Blems can be prevented by exercising proper precaution. Nobody burns their tubes on purpose, but if it happens don’t turn them off. Cap the lenses, place in a box void of all light, let it run until the battery dies. Repeat until the burn works itself out as much as possible. Heavier burns/blems likely won’t go away. This is essentially scar tissue in the intensifier, for lack of a better analogy.
The image below demonstrates a dark spot which looks like debris. Factory dark spots can be a variety of shapes but are most commonly seen as a circular spot. Irregular shapes, lines, and splotches do occur and do not automatically mean a dark spot is debris or a blem. If there is a spot in your tube not reported on the data sheet consult your vendor to have them perform and inspection and dark spot defect test using their test equipment. A properly equipped facility will have a test set and specialized camera system which performs these functions and more.
Many use the two terms, dark spots and user induced blemishes interchangeably, as they either don’t know any better or have heard it elsewhere referenced as such before. Whatever the case, they do not mean the same thing. They are different in manufacturer definition. Dark spots are the proper nomenclature for factory spots as referenced in the manufacturer image quality performance specification parameters.
Why is this important other than semantics?
When researching your device it’s important to understand what you’re purchasing. As well as understand if there’s a possibility for alleviating blems if you encounter one. If it’s a burn maybe you can black box it away. If it’s debris try having it serviced to see if it helps. We recognize some of you may gamble in purchasing an eBay tube or something with an unknown history or otherwise salty device. While we don’t really recommend it, we can at least provide you with some knowledge to better understand what you’re purchasing.