Index of /~jhollan/gw/activities/img_albeda_cosine

	Name	Last modified	Size	Description
	Parent Directory		-
	visual_up.jpg	2012-06-21 16:00	5.5M
	visual_down.jpg	2012-06-21 16:00	5.4M
	tosun_sunlit.jpg	2012-06-21 15:26	6.8M
	tosun_shadow.jpg	2012-06-21 15:26	6.9M
	sunlit.jpg	2012-06-21 15:28	5.5M
	solar_up.jpg	2012-06-21 16:05	6.3M
	solar_down.jpg	2012-06-21 16:05	6.4M
	shadowed.jpg	2012-06-21 15:28	5.5M
	configuration.jpg	2012-06-21 15:28	4.7M
	char_up.jpg	2012-06-24 15:00	4.7M
	char_down.jpg	2012-06-24 14:59	4.8M
	320/	2013-01-14 23:41	-

Visual albedo of biochar:

Verifying lambertian (cosine) sensitivity of a luxmeter sensor. Configuration at 0.1 rad ‘angular height’ (84.3° from normal incidence). Two details of measurement. The stick casting a shadow onto the sensor is to be held just a bit shifted when the sensor is in sunshine, to block a piece of the sky close to sun – this way, the bias is minimised:

Direct oblique sunshine, as apparent, is (18.1-7.8=10.3) klx.

A and an analogous measurement at normal incidence, using a hand to cast the needed shadow:

Direct normal sunshine, as apparent, is (101.5-14.7=86.8) klx. In the preceding, oblique case at (π/2 - 0.1) rad, it should be 10× less, but it is not so little. We may guess that the sensor reports some 1.2× larger signal than it should, at this oblique direction.

Would it do any problem? For scenes of uniform luminance, the relative amount of light from such a direction, which is very far from a perpendicular one, is little anyway. The integral of effective space angle from 0 to x=π/2 - 0.1 is pi*(1-cos(2*x))/2 = ~3.110, just 1 % less then π. A 20 % error in the measurement of such extremely obliquely incident light can be mostly neglected. Of course, less oblique directions are more important and are to be inspected in a similar way.