General > General Technical Chat
Video on planned obsolescence.
tooki:
--- Quote from: BrokenYugo on April 04, 2021, 07:23:08 pm ---You're overlooking that this resistor has to get white hot, more mass/area=lower temperature=lower luminous efficiency.
--- End quote ---
Not to mention that the resistor would rapidly grow in size, to where it wouldn’t fit inside the bulb.
People forget just how long the filament is: it’s not a coiled wire. It’s a coiled coil of insanely thin wire, so it’s far longer than it appears. If we increased its diameter, we’d have to increase its length too...
SilverSolder:
If a filament is suspended in a perfect vacuum, do we agree that the only way it can lose heat is by radiation? (IR + light)
Wolfram:
--- Quote from: SilverSolder on April 04, 2021, 07:41:19 pm ---If a filament is suspended in a perfect vacuum, do we agree that the only way it can lose heat is by radiation? (IR + light)
--- End quote ---
Radiation is by far dominant, and there will also be some conduction down the lead-in wires. Part of the IR from the filament is re-absorbed by the outer glass envelope, some of which is re-radiated and some of which is conducted to the socket. An energy balance diagram for gas-filled lamps can be found at http://lamptech.co.uk/Documents/IN%20Operation.htm , along with other relevant info.
james_s:
There were a few incandescent lamps that employed an IR reflective coating on the inside of a specially shaped capsule to reflect radiated heat back at the filament. The challenge is that it requires extremely precise placement of the filament in the focal point for that to work, and that means that it has to be relatively rigid. The early attempts used a filament that was extremely brittle and often did not survive shipping from the manufacture. More recently around 12 years ago I got a few 60W equivalent lamps that used something like 38W. They used a small quartz capsule with a compact tightly wound filament. They worked well, but still couldn't compete with even the early LED bulbs.
They really did make attempts to push incandescent as far as possible. There were long life lamps, vibration resistant lamps, high efficiency lamps, low cost lamps, lamps optimized for heat output, lamps optimized for high color temperature, lamps optimized for any one characteristic for specific applications. With over 100 years of development it is a very mature technology, at some point there just wasn't much further it could be taken.
tooki:
Tangent about IR reflection: did you know that halogen reflector bulbs come in two kinds? One uses an aluminum reflector coating, and reflects all the IR energy. This means the fixture doesn’t get hot, but the object lit by the bulb does. The other kind uses a dichroic mirror coating, which reflects visible light, but lets IR pass through, keeping the lighted object cooler, but requiring a fixture that can handle the heat.
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