Lux Meter readings of a LED conversion

[75RR]

This has taken a little while.
Original idea/encouragement from Ichthyophthirius See: http://www.photomacrography.net/forum/v ... c&start=15
Subsequent help/hand holding from Crater Eddie See : viewtopic.php?f=12&t=996

The LED is a XML-U2 : http://www.cree.com/~/media/Files/Cree/ ... ampXML.pdf

This is the Dimmer/controller I used: http://www.ebay.com.au/itm/20A-DC9V-60V ... AQ:AU:1123

The resistor to bring the 5A of the power Supply down to the 3A of the LED : http://www.ebay.com/itm/181755869510

First image shows the heatsink, it is designed to occupy the same plane as the Element in the Tungsten Bulb and therefore make it possible to achieve Köhler.
I found it best to focus the element at the Front Focal Plane, it is easier to see there. An angled slide was used to view the element.
Offsetting the element so its edge can be seen facilitates focusing. That is the best way to proceed. Offset, focus then center.
Third image shows the crenelated lower edge of the LED's element focused on the Condenser Iris (Front Focal Plane), one of the requirements to achieve Köhler.

This setup works with both LED (with a resistor) and with the incandescent tungsten bulb. (I had no dimmer and the 2V steps of the power supply were a little long.)

LED conversion is on a Zeiss 12V 60W illuminator




Voltage and Current Outputs on the LED at 6v, 8v, 10v and 12v settings.
All Tests run from lowest Dimmer setting to highest.

Nominal 6V setting on Power Supply
IN Range 7.7V - 7.31V
OUT Range 1.79V - 3.38V
OUT Range (Incl. Resistor) 1.92V - 2.78V
Current Range 0.01A - 0.29A

Nominal 8V setting on Power Supply
IN Range 10.5V - 9V
OUT Range 2.12V - 9V
OUT Range (Incl. Resistor) 1.93V - 3.12V
Current Range 0.03A - 1.55A

Nominal 10V setting on Power Supply
IN Range 13.4V -11.3V
OUT Range 2.24V - 11V
OUT Range (Incl. Resistor) 1.98V - 3.2V
Current Range 0.05A - 2.12A

Nominal 12V setting on Power Supply
IN Range 16V - 13.2V
OUT Range 2.3V - 12.9V
OUT Range (Incl. Resistor) 2V - 3.3V
Current Range 0.07A - 2.74A

Note: I find the 6V setting is low enough but range is too narrow. 8V, 10V and 12V settings are too bright at lowest settings.
It is not the highest setting that is the problem but the lowest setting.
When centering or focusing the Field Iris, lowest settings are too bright to look at except for the 6V.

Visual representation of the figures above.
Note how the range of Nominal 8v(a) in dark blue contracts and shifts downwards when using two Resistors in Series.

[75RR]

UPDATE:
Luxmeter arrived, slightly larger and older than I thought. In perfect working order.
Only caveat is that it is not sensitive enough, scale in 5 Lux steps min.
Range is: 0 - 300, 0 - 1000 and 0 - 3000 Lux
Note: Battery for scale only, it does not use batteries.

Luxmeter pdf: http://tmi.yokogawa.com/files/uploaded/ ... alog_1.pdf



Looking through the candlepowerforums it would seem that the best way in terms of accuracy and relevancy is to remove both the LED and the Tungsten Bulb from the microscope and shine them on the Photocell from precisely 1 meter away.
That is because at 1 meter distance, 1 Lux (amount of light a surface area receives) is equal to 1 Candela (a measurement of intensity).
This will allow a repeatable and consistent test that will be relevant to others.

The idea is to test the Tungsten Bulb powered directly by the Power Supply using AC at 4V, 6V, 8V, 10V and 12V and then repeat the test using DC.
I will also test the Tungsten Bulb powered by the PWM Controller/Dimmer to see if the maximum intensity varies.
Similar testing will be done on the LED in order to ascertain lowest (startup) and highest light intensity. Results will be compared to the Tungsten Bulb.

Work table converted to a provisional Testbed.


Tungsten Bulb measurements.


Unfortunately while I am able to view and photograph at the 4V setting directly from the Power Supply, using up to 25x in Brightfield, the Luxmeter is unable to measure the Candela at these settings at 1 meter as it is only able to measure 5cd minimum.
A similar restriction occurs at 6v using the Controller/Dimmer.

Note: It has been suggested that I take measurements at 1/2 a meter (which should allow the Luxmeter to register the Lux at 4v and 6v) and divide the results by 4 (inverse square law), as this would maintain the results in Candela.
I have not tried this as yet.

The LED.
These are some prelimenary results using the Controller/Dimmer with the LED and setting the Power Supply to 8V, 10V and 12V.

8V
Start up .........5cd
Max ...........180cd

10V
Start up ......7.5cd
Max ...........230cd

12V
Start up .......10cd
Max ...........270cd

Note 1: Ambient light was subtracted from the Luxmeter readings during test.
Note 2: Power Supply actual output at Nominal 12v is 13v on AC and 16v on DC.

Provisional Conclusions:

The minimum startup Candela from the LED is much too bright.
Curiously the Max Output is not much more than what the 12v Tungsten Bulb provides.
It would therefore seem that for low objectives in brightfield and Oblique, Candela below 5 is more than adequate.
Higher Candela only needed for medium and high objectives, especially if using light intensive illumination techniques like Phase.

Have gone back to using the Tungsten Bulb while I attempt to source another Controller/Dimmer that will allow a lower minimum Candela startup.

[zzffnn]

Very nice work!

[75RR]

Thanks zzffnn. Hope you find it useful.

[billbillt]

Hi 75RR,

Thanks for the interesting and detailed report on the light readings.....

[75RR]

Thanks billbillt.

[Crater Eddie]

Good write-up.
It has been an interesting and educational project.

[75RR]

Good write-up.
It has been an interesting and educational project.

Many thanks, could not have done it without you.

[gekko]

Nice work.