DMM Diode Test - What exactly?

[dav]

I measured a red LED with my 87V and I got 1.830V.
I added another meter in series to measure the current and I read 0.98mA so I thought ok, the 87V estabilishes enough voltage to force 1mA in the diode and returns that value.

Then I tested a second  LED (white) and here what I got: 2.667V  0.85mA
so my previous hypothesis is wrong.

Moreover I tried same LEDs with another meter, now using the 87V as an ammeter.
Here the measurements:
red LED  1.805V  0.67mA
white LED  2.580V  0.31mA


So what does the meter exactly measure in diode test? 

[joseph nicholas]

If you read the manuel it usually tells you the ohmic resistance applied.  I think different meters use different ohmic resistance.  Whats more your battery may have something to do with it.

[TimFox]

The normal diode test in a DMM applies a constant current (value varies with model) to the DUT and measures the voltage across it.  The maximum voltage is limited, so it may not work well with a (reverse-biased) zener diode or sometimes with a high-voltage LED.

[Seekonk]

I measured a meter on ohms and OC voltage was 2.97, so you are likely bumping at the edge of some meters providing a constant current.  Your test tells you something, but not much.

[Wytnucls]

Most meters use a constant current of about 1mA for the diode test (the chosen current is usually in the range of 0.4mA to 2mA). The voltage across the diode or LED is then displayed.
LED datasheets use a current of 20mA to characterize the forward voltage drop.
Open circuit voltage varies from as low as 1.8V for some meters, up to 15V for a few others.
All of them can measure the drop out voltage of a normal diode (0.6V). Few are designed to light up a LED and measure its drop out voltage.
Connecting a current shunt in series in the test circuit to measure the constant current makes things even more difficult for the meter, increasing total drop out voltage.
As an example, testing a diode, gives a drop out voltage of 0.6V. With another meter introduced in the circuit to measure current, the displayed drop out voltage becomes 1.6V.
With a white LED, the drop out voltage is a lot higher (2.6V). Compliance voltage can be exceeded and the meter's constant current source cannot maintain the required 1mA.
I suggest you go and read about constant current sources.