OSRAM Parathom 8W LED Lamp

[jtr1962]

Where/How did you come up with this assumption/conclusion?

The 4 watt number is based on the maximum temperature rise of a heat sink the size of a small A19 bulb needed to keep LED junction temperatures safe.

The property of 1-die high power LED is typically 2.8-3.2V by default. The input power should equal to (number of LEDs)*(drive current)*(voltage of 1-LED).

No, you need to convert the AC to DC first, and that process is less than 100% efficient, so you need to account for that. Also, the LED drive current is irrelevant here. The only number we're interested in as far as the LED goes is the wall-plug efficiency at whatever current it is being driven at. This is the percentage of input power which comes out as light energy. The rest of the input power ends up as waste heat.

Let's calculate reverse to look at the LED driving condition and conclude again.
Lamp power * ballast efficiency = total LED power (8 * 0.85 = 6.8W)
Total LED power / number of LEDs / voltage of 1-LED = drive current (6.8 / assume 3 LEDs / 3V) = 756mA
If Osram increased the number of LEDs, they can lower the driving current to get lower LED junction temperature.
If the lamp only has 3 LEDs then it's probably too hot to last 25k hrs; but if they increase to 6 LEDs (halved drive current) then there is chance the LEDs can last 25k hrs.

Yes, you can increase the number of LEDs, drive them at lower currents, and get increased efficiency along with increased lifetime. The problem is it wasn't as economically viable to do this at the time this thread was started because emitters cost more.

As far as figuring out the waste heat here, a good rule of thumb is to use (input power - output lumens/300). The emitted efficacy of a typical LED spectrum is roughly 300 lumens per watt. Let's say this lamp puts out 500 lumens. That means 500/300 = 1.67 watts comes out as light. The rest is waste heat. If 8 watts goes in, then that means 8 - 1.67 = 6.33 watts is waste heat. This is obviously too much for such a small sized heat sink to handle, so LED junction temperatures will be too high. That's why the lifetime is only 25K hours instead of 70K to 100K hours.

LED bulb cost has fallen since this thread was started, but lifetime is much lower than it could be due to the thermal issues. As I said back in 2010, the thermal issue will continue to be a problem until LEDs hit efficacies of roughly 200 lm/W or more.

 

[Anders Hoveland]

OSRAM Parathom 8W LED Lamp

The tint is "warm white", quite pleasant to the eye IMO. No ugly tint at all.

I have this bulb. It might just be me, but the light quality from this particular brand of bulb seems better than all my other various LED bulbs, including the same 40 watt shape from other brands. I do not know whether it is some slightly better phosphor, or if the CCT is just somewhere between 2700-3000. Perhaps it is just 2800, and I prefer this color of light over 2700 or 3000 from an LED.

So yes, I agree, best color tint.

 

[BM_lol]

Oops didn't realize this is a 2010 thread

No, you need to convert the AC to DC first, and that process is less than 100% efficient, so you need to account for that. Also, the LED drive current is irrelevant here. The only number we're interested in as far as the LED goes is the wall-plug efficiency at whatever current it is being driven at. This is the percentage of input power which comes out as light energy. The rest of the input power ends up as waste heat.

The terms used wasn't clarified so that's why we have confusion. I analyzed the power based on LEDs only without driver.
'Input power': I meant LEDs only (doesn't include driver)
'Lamp power': System lamp power (includes driver)