Page 300 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

290 CHAPTER 11 Vehicle Sensors and Lighting
11.2.1 Lighting theory
This section is a condensed version of a technical paper by Cyril Poissonnet of Remote Ocean Systems, which is available for download from the ROS web site. There is much misconception about lighting. Most household lights are measured in wattage—which is a measure of electrical power consumed (not light power generated). Thus, wattage is a proper metric sizing the power source but has no direct correlation to the actual amount of light produced. Closer examination of lighting parameters must be considered to achieve the mission’s lighting requirements.
Lighting power is measured in SI units as the candela (as originally defined, a candela is the light intensity given off by one candle, although that definition was later refined). Visible light is further divided into the various wavelengths as depicted in Figure 11.1. Light is thus discriminated into both quantity and quality.
11.2.1.1 Quantity
The unit candela is an omnidirectional unit that does not take into account directionality and is independent of distance. As with sound, light is subject to a concept known as “spreading loss.” As light spreads away from its source, the power is continually spread over a wider area, thus deplet- ing its power per unit area. Thus the term “luminous intensity,” or “lumen,” arose as a more defini- tive metric since it measures total flux intensity from a light source. “Lux” is then defined as lumens per unit area. By standard convention, lux is referenced at a range of 1 m (defined as lumen/m2).
A lighting product is specified by its lumen output and its color temperature, along with a direc- tionality factor for the beam angle. A floodlight illuminates an entire scene while a spotlight focuses on an individual item within the scene. With a constant lumen output from the lighting source, the actual light incident to the surface of the target (its lux) is much higher for the spotlight than the floodlight. The beam is shaped by the reflector, thus focusing and intensifying the lumens onto a discrete surface (Figure 11.2). And with spreading loss, the beam loses intensity as it propa- gates from the light source (Figure 11.3).
FIGURE 11.1
Gamma rays
Radio
Ultra violet
X-rays
Visible spectrum
Wavelength
Infra red
Microwaves
400 nm
700 nm
The visible light spectrum.
Violet Blue
Green
Yellow Orange
Red