https://www.getiangroup.com/resources/what-is-a-cob-light-and-when-would-you-use-it.html
A COB light is a type of LED light, with COB standing for Chip-On-Board. COB lights are essentially an array of LED chips that are tightly packed together and bonded to a substrate such as silicon carbide. In effect this creates one large LED chip that has excellent uniformity of brightness, thus making it ideal for photographers and filmmakers.
When most people think of LEDs they probably think of the small singular lights that they experimented with at school when learning about electronics. Those types of singular round LEDs are called DIP LEDs, or Dual In-Line Package. They are also the kind of LEDs you might find in a control panel to indicate when a switch is on or off. DIP LEDs are cheap, but they are bulky and not very powerful.
The Godox SL-60 COB light.
Taking a 10mm x 10mm square LED array as an example, you would only fit 9 DIP LEDs on the square. You would fit 342 COB LEDs in the same space, though, so you can instantly see the advantage in a light that is designed for studio photographers or filmmakers. A much brighter light source can be created using a much smaller light.
If the light is to appear in a shot itself, a COB appears as a single light source, where older LED panels that were constructed of hundreds of DIP LEDs all lined up, would often show a zebra pattern, along with far less panel uniformity.
COB lights are also relatively simple in the way that they work, and this created a robust and reliable design. With a large array of older DIP LEDs, each individual LED would have its own circuit to power it. A COB only has a single circuit, with the whole COB panel acting as a single light, despite containing multiple small LED panels.
One of the downsides to a COB light, especially in a studio scenario, is the heat that is generated. With such a huge amount of light energy created in a small area, the heat is very localized and requires passive and active cooling. Passive cooling in COB studio lights is done by huge heatsinks that surround the front-mounted COB panel. You can see it clearly in the photo of the Godox SL-60.
Active cooling if often also necessary, and this means there will be a fan inside the head. In a stills photography studio a fan is a small problem, but they can be an issue for filmmakers if you need to record sound. A single COB light is unlikely to cause problems, but if you had a huge number of them it would become something to be aware of. Also worth noting is that the cheaper the lights you buy, the cheaper and noisier the fans will be.
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