Diagram showing the placement of Picture framing glass in a picture frame

What is the function of picture framing glass?

Picture framing glass is specially treated glass used in a frame to create a transparent barrier to protect the artwork. Available with different levels of UV Filters; light reflection, light transmission and UV protection, understanding these filters is key to choosing the right picture framing glass for each framing project.

Cross section diagram of an assembled frame — Cleaned Picture framing glass, mount, picture and backboard sit in the frames rebate

Glass has 2 main features

The overall strength of the glass – Depending on the size of the glass is it thick enough to support itself? If your glass is over 1000mm x 1000mm you should look at increasing the thickness of the glass from 2mm to 3mm.
The level of UV filters – UV protection, Reflection and light transmission are the units used and measured to qualify how protective a particular glass is. A high UV qualified glass will ensure maximum protection for a long period of time.

Here are the UV filters explained in detail:

What exactly does “light reflection” mean?

‘light reflection’ describes the light that gets reflected back to the viewer. To explain it in easy terms, light changes speed when entering a glass surface, which results in not all of the light getting through to the artwork. On uncoated glass, around 8-10% of light is reflected (~4% from each glass surface side). Anti-reflective coatings manipulate these incoming light waves, making it easier for the visible light to travel through.

What exactly does “light transmission” mean?

Light transmission is the amount of (usually visible) light that gets through the glass, onto the artwork. The higher this number, the more light reaches the artwork, ensuring better visibility. Both reflection and transmission percentages are usually adjusted for viewing glass at an 8-degree angle (Because our coatings are optimised for being looked at directly from the front or from angles close to that)

How does the UV protection work?

There are two ways that UV protection can work. The first is a useful side-effect of anti-reflective coatings. These coatings that I described in “Light reflection meaning” work very well to improve transmission within the visible light region of the light spectrum, but a side effect of the process is that the coatings reflect the majority of the light in the UV light spectrum. This is the case Low Reflection Invisible glass & our UV Clarity glass. The UV block is achieved through reflection (reflecting 70% and 92% of UV light respectively, instead of transmitting it through to the artwork).

For our 99% protection glass products(UV Museum & UV plain), the protection is achieved with an additional production step – a chemical coating layer that provides a museum-grade UV protection layer. In this case, the UV block is achieved through absorption. You can watch this link to see more about the chemical coating process in this video