Edge lit vs backlit frames: how the light gets to your picture

When people compare edge lit vs backlit frames, the real question is where the LEDs sit. In an edge-lit frame, a thin row of LEDs along the edges shines sideways into a clear acrylic sheet that sits behind the picture. In a direct-lit frame, the kind most people picture when they hear “backlit,” a grid of LEDs sits straight behind the picture and shines forward. Both light the image from behind; they just get the light there in different ways.

Much of what is written about the two is aimed at sign makers. This guide is for someone choosing a lit frame for a photo, a movie poster or a piece of art at home. For the other ways a frame can glow, such as LED neon borders and wall halos, see our overview of illuminated frames.

First, a note on the word “backlit”

“Backlit” gets used two ways, and it causes confusion when you shop:

• In everyday use, backlit means the light comes from behind the picture. By that definition, edge-lit frames are backlit too, because their light panel sits behind the print.
• In engineering use, backlit is split into edge-lit (LEDs at the edges of a light guide) and direct-lit or full-array (LEDs spread across the back). The same split is used for flat screens, as Wikipedia’s article on LED-backlit LCDs describes.

So a product can honestly be called a “backlit poster frame” and still be edge-lit inside. Our own Lumi Frames Lightbox is exactly that: a backlit poster frame that uses an edge-lit LED light guide panel. In the rest of this guide, “backlit” means the direct-lit design, so the comparison is clear.

How an edge-lit light guide panel works

A light guide panel (LGP) is a sheet of clear acrylic with LEDs pointed into its edges. Most of the light that enters an edge never leaves through the faces of a plain sheet. It glances off the front and back surfaces and bounces its way across the panel, an effect called total internal reflection. Wikipedia’s article on total internal reflection puts the critical angle for acrylic to air at about 42 degrees and gives the edge-lit LED panels that backlight computer monitors as an example.

To get the light out where you want it, the panel is patterned with tiny dots, etching or diffusing particles. Each dot knocks some of the trapped light forward, toward the picture. The trick is in the spacing: as the Wikipedia article on backlights puts it, the density of bumps increases farther from the light source, so the far middle of the panel glows as brightly as the part next to the LEDs. Some acrylic makers build diffusing particles into the sheet itself. Polyvantis, which makes ACRYLITE edge-lit sheet, claims uniform illumination with “no visible contours of light sources.”

A white reflector behind the panel sends stray light back toward the front. Plaskolite’s product page for its OPTIX-L Edge Lit sheet recommends reflective tape and a white backing behind the image to direct the light forward.

How a direct-lit LED array works

A direct-lit frame is simpler. LEDs are laid out in rows or a grid on the back panel, all facing forward. Some distance in front of them sits a diffuser, a sheet of frosted acrylic or film, and the picture sits on top of that.

Each LED throws a cone of light. The diffuser can only hide the individual LEDs if those cones overlap before they reach it, and that takes distance. A lighting patent for LED sign cabinets, originally assigned to GE Lighting Solutions, describes this with a “box sign depth factor”: the distance from the LEDs to the sign face divided by the spacing between neighboring LEDs. Its worked example has LEDs 4 inches apart sitting 4 inches behind the face, a factor of 1 (US9564070B2). The patent describes factors from about 0.5 to 1.25 in some designs, but the principle holds: more LEDs, spaced closer together, let the box get thinner.

Edge lit vs backlit at a glance

Evenness and hot spots

Evenness is the thing you notice first. A patchy glow makes even a great poster look cheap.

Edge-lit frames are even by design, but only as even as their dot pattern. A well-made panel glows the same from corner to middle. A poorly made one shows a brighter band near the LED edge or a dimmer patch in the center, which you notice most on large, pale areas such as sky.

Direct-lit frames are even if they are deep enough for their LED spacing. Squeeze the box too thin and each LED shows through as a bright circle. That trade-off is why many slim “backlit” frames are edge-lit inside. Wikipedia’s article on LED-backlit screens lists uneven light, “bright hot spots or dark patches,” as a known drawback of LED backlighting in general, so the design matters more than the label.

What you put in the frame also affects how even it looks. Thicker, printed-both-sides “light box” posters hide small variations, and they glow more richly too; see how double-sided movie posters are made.

Depth, weight and how it hangs

Depth is where edge-lit wins clearly. With no air gap to fill, an edge-lit frame can sit close to the wall like an ordinary picture frame. The Lumi Frames Edge is about 0.6 in deep, and the Lumi Frames Lightbox, sized up to a 27 × 40 in movie one-sheet, is about 1.1 in deep.

Weight works differently. A light guide panel is a solid sheet of acrylic, so a big edge-lit frame carries that sheet’s weight. A direct-lit box is mostly air, but it needs a deeper housing to hold that air. Either way, use hanging hardware rated for the frame and a stud or proper wall anchor for poster sizes.

Brightness, power and heat

Because a direct-lit box can hold as many LEDs as its back has room for, it is the usual choice when a very large or very bright face is needed, such as a sign seen from across a street. An edge-lit panel is limited by how much light its edges can take in, and that light has to travel farther across bigger panels. For a picture in a living room, that ceiling rarely matters, because a frame a few feet from your sofa does not need sign-level brightness.

Power use follows the number of LEDs and the brightness setting, not the design itself. Heat follows where the LEDs sit. According to ENERGY STAR’s guide to LED lighting, managing heat is generally what matters most for an LED’s performance over its lifetime. Edge-lit designs concentrate the LEDs, and their heat, along the frame edges, where a metal frame profile can help carry it away. Direct-lit designs spread the heat across the back. For more on running lit frames safely, read our LED frame safety guide.

Cost

In flat screens, edge lighting is the cheaper design and full-array lighting the more expensive one, according to Wikipedia’s backlight article. Frames are simpler than screens, and the lighting method is only one part of the price. The things that tend to move a lit frame’s price are its size, the quality of the light guide or diffuser, the color rendering of the LEDs, and extras such as tunable white, dimming and remotes.

The better design for photos, posters, film and glass

• Photos and art prints. Edge-lit. It keeps the frame slim enough for a desk or a gallery wall, and small sizes can run on a battery.
• Movie posters. Edge-lit handles one-sheets well, especially light box-ready posters. The Lumi Frames Lightbox is built for this.
• Backlit film and transparencies. Either works. These are made to be lit from behind, so evenness matters more than raw brightness. See how to display film cells for small formats.
• Stained glass. Dense, colored glass swallows a lot of light, so it needs a deep direct-lit box or a bright edge-lit panel. How to backlight stained glass walks through both routes.
• Engraved or clear acrylic art. Edge lighting is the natural fit, because engraved lines break up the smooth surface and let the carried light out, so the design itself glows. See how to light up acrylic art.

How to tell which one you are buying

1. Read the light source line in the specs. “Light guide panel,” “LGP,” “edge-lit” or “acrylic light panel” means edge-lit. “LED array,” “LED modules” or “direct-lit” means LEDs behind the picture.
2. Check the depth. A frame around an inch deep with a large face is very likely edge-lit.
3. Look for a photo of the frame lit with no picture inside. An empty, evenly glowing panel tells you more than any styled photo.
4. Check the white options and dimming. Tunable white and smooth dimming matter more to how your picture looks than which design is used.
5. Match the power to the spot. If there is no outlet within reach, a rechargeable edge-lit frame saves you running a cord.

Still weighing options? The Lumi Frames comparison chart lists the light type, depth and power source of each of our frames.