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LED Characteristics

LED datasheets commonly provide:

  • \$ I_{v}\ [cd] \$ Light Intensity vs \$ I_{f}\ [A] \$ Forward Current
  • \$ V_{f}\ [V] \$ Forward Voltage vs \$ I_{f}\ [A] \$ Forward Current
    • (Often, the inversion of these axes is depicted)

As an LED is driven at higher forward currents, its light intensity generally increases (non-linearly).

As an LED is driven at higher currents, its forward voltage increases (non-linearly).

LED efficiency

 

[ \$ I_{v} \$ ] vs [ \$ I_{f} \$ ]

LED efficiency is often described in terms of:
Output \$ I_{v} \$ Light Intensity \$ [cd] \$ vs
Input \$ I_{f} \$ Forward Current \$ [A] \$.

[ \$ \frac{ I_{v} }{ I_{f} } \$ ] vs [ \$ I_{f} \$ ]

Datasheets often suggest a tested operating state, which incidentally often corresponds to the peak \$ \frac{ I_{v} }{ I_{f} } \$ vs \$ I_{f} \$ efficiency.

Designers may utilize a forward current above the suggested tested operating state, typically to achieve increased maximum light intensity at the expense of direct [ \$ \frac{ I_{v} }{ I_{f} } \$ ] efficiency.

PWM methodologies, for example, reduce actual light intensity in proportion to the ratio of the duty cycle, but may achieve gains elsewhere, (such as through reduced temperature cycling, increased perceived light due effects of the Weber-Fechner Law), and simple dynamic control of intensity during operation).

It is common for designers to partly compensate for the reduction in maximum actual light intensity due to duty cycle by driving the LED at a higher forward current.

Issue

The level of change to [ \$ \frac{ I_{v} }{ I_{f} } \$ ] efficiency is not always easily distinguished on a datasheet, however, as it is depicted as the slope of the non-linear [ \$ I_{v} \$ ] vs [ \$ I_{f} \$ ] graph.

[ \$ \frac{ I_{v} }{ I_{f}\ •\ V_{f} } \$ ] vs [ \$ I_{f} \$ ]

As stated before, as an LED is driven at higher currents, its forward voltage increases (non-linearly). Increases to forward voltage are relevant, as they will increase the minimum bus voltage necessary to supply the LED. [This is especially relevant in limited power storage (battery) applications.]

This thus has an impact on a more robust consideration of efficiency:
Output \$ I_{v} \$ Light Intensity \$ [cd] \$ vs
Input \$ ( I_{f}\ •\ V_{f} ) \$ Forward Power \$ [W] \$.

Output light intensity and forward voltage are both relational to input forward current, and both of those relationships are provided on LED datasheets already.

Issue

Where [ \$ \frac{ I_{v} }{ I_{f} } \$ ] efficiency requires eyeballing the slope of the non-linear [ \$ I_{v} \$ ] vs [ \$ I_{f} \$ ] graph, [ \$ \frac{ I_{v} }{ P_{f} } \$ ] efficiency requires mentally dividing the non-linear [ \$ I_{v} \$ ] vs [ \$ I_{f} \$ ] graph with the non-linear [ \$ V_{f} \$ ] vs [ \$ I_{f} \$ ] graph. This makes part and operating state selection with respect to [ \$ \frac{ I_{v} }{ P_{f} } \$ ] efficiency especially challenging.

Question

Why aren't Efficiency vs Forward Current graphs depicted in datasheets, such that the nonlinear relationships and salient operating points are much more transparent:

  • \$ \frac{ I_{v} }{ I_{f} }\ [ \frac{cd}{A} ] \$ vs \$ I_{f}\ [A] \$
  • \$ \frac{ I_{v} }{ I_{f}\ •\ V_{f} }\ [ \frac{cd}{W} ] \$ vs \$ I_{f}\ [A] \$
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  • \$\begingroup\$ Because it will vary widely between devices and if the manufacturer specs it then they have to test each device to see if it meets spec, which is not worthwhile on dies that sell for a few cents each. Instead of efficiency is important for a product line they'll spec a single operating point, which is obviously much cheaper to test. \$\endgroup\$ Jun 12, 2023 at 19:14
  • \$\begingroup\$ @user1850479 : LEDs fairly frequently provide \$I_v( I_f )\$ and \$V_f( I_f )\$. Plotting the same data as \$\frac{I_v( I_{f} )}{ I_{f} }\$ and \$\frac{I_v( I_{f} )}{I_f}\ •\ \frac{1}{V_f( I_f )}\$ is programatically quite simple. \$\endgroup\$
    – kando
    Jun 12, 2023 at 20:10

1 Answer 1

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The change in intensity with current is only slightly sublinear; similarly the change in VF is quite small. Therefor the overall change in efficiency with current is not generally a concern as color rendition and cost are watched more.

p.s. "increased perceived light due persistence of vision of the human eye" is not real

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  • \$\begingroup\$ The Broca-Sulzer effect and the Bruecke-Bartley effect consider flicker frequencies below the critical flicker frequency. At around 50ms on-duration and 50ms off-duration, for example, a blinking light source will appear to be brighter than the time-averaged luminance ... in apparent violation of the Talbot-Plateau law (whcih doesn't apply at such low frequencies, so it isn't not a violation.) This can be useful in cases where instrumentation is placed in forest systems for a year's operation to be later picked up in the dark. \$\endgroup\$ Jun 12, 2023 at 20:41
  • \$\begingroup\$ @periblepsis and jp314 : I moved this subtopic here. \$\endgroup\$
    – kando
    Jun 12, 2023 at 20:47
  • \$\begingroup\$ Changed question from "due to effects of persistence of vision" to "due to effects of Weber-Fechner Law)". Added link to citable reference in case of confusion. Will remove if subquestion debunks this theory. \$\endgroup\$
    – kando
    Jun 12, 2023 at 20:49
  • \$\begingroup\$ 50 ms on and 50 ms off is a 10 Hz flashing light. That is not useful for general illumination as is implied in the original question. \$\endgroup\$
    – jp314
    Jun 13, 2023 at 5:10

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