July 01th, 2021
Evelyn: Stephen，shall we continue the conversation about coating from last time?
Stephen: Sure, then let's talk about something interesting this time. Do you know how the coating is plated onto the lens?
Evelyn: I have no idea.
Stephen: Look at Picture 1, this is the device used for coating. Coating’s main composition forms by magnesium fluoride, H4, and aluminum trioxide. The pre-treated lenses are placed on this shower-like disc, and once the device is activated, the machine becomes a vacuum, and the coating is vaporized and applied onto the glass.
Evelyn: Wow, it’s cool!
Stephen: And in pursuit of better results, the coating on multi-coated lenses nowadays is commonly more than ten layers like picture 2 shows. Of course, the types of these coatings vary, as I mentioned to you before.
Evelyn: So is there a difference in the quality of the coating?
Stephen: When discussing light transmission, we talked about the effect of exit pupil on light transmission. The exit pupil is equal to the objective lens diameter divided by the magnification（picture 3）, so we usually think that the smaller the pupil diameter, the lower the brightness and the darker the image we see.
Evelyn: That's true.
Stephen: But even scopes with the same exit pupil can feel the difference in brightness when used, especially in low light conditions such as dawn and dusk. When all other variables are equal, the difference in coating quality can make a significant difference. (picture4)
Evelyn: I see.
Stephen: A first-class scope can provide more than 90% (some can reach to 95%) of the light transmission, which not only means the brightness of the image is improved, but means that you can still use the scope in low-light conditions---the scope's usage time is extended. So the importance of high quality coatings cannot be overstated.
Evelyn: Extending the use of the scope is indeed a great benefit!
Stephen: Exactly. And the sensitivity of the human eye to different wavelengths of light during the day and at night are different. As shown in Picture 5, the curve above shows the sensitivity values of the human eye to different wavelengths of light during the day and at night, and the graph below shows the color of the corresponding wavelengths of light. What can you draw from the chart?
Evelyn: The human eye is more sensitive to green during the daytime, and more sensitive to blue at night. Right?
Stephen: Correct. A first-class coating will even take these factors into account and make improvements.
Evelyn: Amazing! It’s incredible! Is this why some scopes can reach more than 95% light transmission?
Stephen: Yes. Coating encompasses a lot of technology. By the way, the calculation of light transmission is very complicated, and all the results are obtained through hundreds and thousands of tests.
Evelyn: Thank you for sharing today to give me a better understanding of coating.
Stephen: No problem, it’s my pleasure!
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