FIELD LUMINANCE
MAPPING OF THE SKY
PORTABLE, DURABLE INSTRUMENTS FOR SKY LUMINANCE DISTRIBUTION FIELD MEASUREMENTS
ANOTHAI THANACHAREONKIT, PH.D
Post-Doctoral Researcher
California Lighting Technology Center
LUÍS LOMELINO FERNANDES, PH.D
Research Engineer
California Lighting Technology Center
KONSTANTINOS PAPAMICHAEL, PH.D
Co-Director, Professor
California Lighting Technology Center
When sky luminance distributions are measured far from a laboratory
facility, portable and inexpensive instruments are useful. High dynamic
range (HDR) photography has become a practical method for sky
luminance mapping. However, long exposure photography directed at
the sun can cause severe damage to digital camera sensors. To avoid
this problem, a system was designed that integrates an adjustable
shading disk with an HDR camera. Simultaneous global horizontal
illuminance measurements indirectly provide luminance data for the
area of the sky that is occluded by the shading disk. In tests, the system
presented here has achieved relative errors for the zenith luminance
of approximately 10%. An integrated shading disk for an illuminance
meter also is presented that allows diffuse horizontal illuminance
measurements to be easily performed.
HIGH DYNAMIC RANGE (HDR)
BACKGROUND
High Dynamic Range Imaging is a technique employed for sky
luminance mapping, particularly useful when measurements need to
be taken away from a laboratory facility.
Sky luminance mapping using high dynamic range (HDR) digital photography has become
a practical method for measuring the luminance distribution of the sky. However, the
HDR technique requires a sequence of multiple exposure images, and when frequent
long exposures of the sky are taken, the sun can damage the camera sensor. This poster
presents an HDR-based system for measuring the sky luminance that protects the digital
camera’s sensor from direct exposure to the sun. Besides a digital camera, it comprises
an adjustable shading device that attaches to the cylindrical section of a fisheye lens
and an auxiliary illuminance meter for measuring global horizontal illuminance. The
luminance value at the region covered by the disk can later be computed using data
from simultaneous global horizontal illuminance measurements. In addition to the sky
luminance distribution and global horizontal illuminance, measuring diffuse horizontal
illuminance often is required in a daylighting study. For this purpose, the system presented
here also includes an adjustable shading disk attached to the illuminance meter.
SENSOR DAMAGE
INSTRUMENT SETUP
Long exposure images are necessary with HDR imaging; however,
the camera sensor can be damaged when directly exposed to the sun.
The instrument setup for field measurements includes a digital camera with a fisheye lens,
together with an illuminance meter and a luminance meter, for reference.
Image courtesy of Axis Communications
This image shows a photograph taken with a camera where the
image sensor filters have been damaged by the sun.
INSTRUMENTS
SHADING DISKS
The holding bracket of the camera shading disk is
provided by a milling machine universal indicator
holder, to which a custom adjustable stem and
shading disk is attached. This device attaches to the
fisheye lens of the HDR camera and allows the disk
to shield any region of the field of view of the fisheye
lens. The holding bracket of the illuminance meter
shading device is an aluminum holder, attached to the
illuminance meter using the camera tripod screw. For
each of these devices, the arms can be positioned to
shade the sun in any direction. Three interchangeable
shading disks, of three different sizes (2.2 cm, 3.2 cm,
and 3.8 cm), can be mounted on the arms.
SHADING DISK
FISHEYE LENS
LENS CONVERTER
DIGITAL CAMERA
SHADING DISK
TRIPOD
ILLUMINANCE
METER
DIGITAL CAMERA & FISHEYE LENS
A Nikon Coolpix 5400 digital camera with a fisheye lens
was used to capture the digital images of the sky.
To obtain the HDR images and sky luminance distribution,
a sequence of 16 exposures, from 1 / 4000 to 8 seconds,
was taken, with the aperture set to f / 7.9. Each set of
images was merged using the Photosphere image builder
program to create an HDR image. Before sky luminance
measurements, the HDR imaging system was calibrated
under controlled lighting conditions.
OTHER PHOTOMETRIC INSTRUMENTS
A Konica Minolta CL-200 Chroma Meter was used to
measure horizontal illuminance, and a Konica Minolta
LS 110 spot luminance meter was used to measure
zenith luminance.
TRIPOD
TESTING PROCEDURE
Instruments were tested under clear sky conditions.
The angular height of surrounding obstructions was
less than 10 degrees.
1. Instruments placed on tripods and leveled.
2. Global and diffuse horizontal illuminance
measurement taken.
3. Sixteen digital camera exposures taken, each
simultaneously with a zenith luminance measurement.
Global and diffuse horizontal illuminance measurements
repeated every four camera exposures.
RESULTS
In this example, the luminance values obtained with the luminance meter
are relatively stable around an average of 2,560 cd / m2. After processing
the 16 digital images, the luminance mapping gives a zenith luminance of
2,383 cd / m2. Average global horizontal illuminance was 7,020 footcandles,
and average diffuse horizontal illuminance was 1,125 footcandles.
STARTED 4:15 P.M.
ENDED 4:18 P.M.
MEASUREMENT TIME
cd / m2
N
W
3700
E
3300
Average luminance
using meter
2700
S
Luminance
using meter
HDR luminance
N
W
3300
2700
E
1300
S
NUMBER OF LUMINANCE MEASUREMENT:
HDR CAMERA SHUTTER SPEED:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1 / 4000
1 / 2000
1 / 1000
1 / 500
1 / 250
1 / 125
1 / 60
1 / 30
1 / 15
1 / 8
1 / 4
1 / 2
1
2
4
8
LM SHUTTER SPEED:
NUMBER OF ILLUMINANCE MEASUREMENT:
1
2
3
4
5
7250
fc
N
W
E
7150
Global horizontal
illuminance
7050
Average global
horizontal illuminance
6950
S
6850
6750
NUMBER OF ILLUMINANCE MEASUREMENT:
1
2
3
4
5
1500
fc
N
E
W
1400
Diffuse horizontal
illuminance
1300
Average diffuse
horizontal illuminance
1200
S
1100
1000
NUMBER OF ILLUMINANCE MEASUREMENT:
1
THIS PROJECT WAS FUNDED IN PART BY THE CALIFORNIA ENERGY COMMISSION’S
PUBLIC INTEREST ENERGY RESEARCH (PIER) PROGRAM
2
3
4
5
CALIFORNIA LIGHTING TECHNOLOGY CENTER
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