Do It Yourself Solar Energy
Image sensors are capable of responding to a wide variety of light sources, ranging from X-rays through to infrared wavelengths, but sensors used in digital cameras are tuned largely to the visible range of the spectrum that is, to wavelengths between 380 nm and 700 nm. Sensors rely on semiconductor materials to convert captured light into an electrical charge. This is done in much the same way as a solar cell does on a solar-powered calculator. A semiconductor is formed when silicon, which is a very poor conductor, is doped with an impurity to improve its conductivity hence the name semiconductor. Light reaching the semiconductor with wavelengths shorter than 1100 nm is absorbed and a conversion from photon to electrical charge takes place. This happens when the light provides energy which releases negatively charged electrons from silicon atoms. Note that light with wavelengths shorter than about 380 nm is essentially blocked by the silicon while light with wavelength greater than...
There are two basic operating types for non-camera light meters. The first type is photovoltaic meters. These meters capture the energy from the light in the scene and convert it to a very low electrical charge that is measured with a voltmeter. The second type is resistance meters. They use various light-sensitive materials that alter electrical resistance based on the amount of light present, and they use an ohmmeter to read the in-line resistance to measure the light. Regardless of how a meter measures light, its function is the same The meter establishes the level of light either falling on the meter or reflected from the subject, and it uses that energy measurement to calculate an effective exposure. The meter recommends an f-stop and shutter speed combination based on a specified ISO.
The first modern light meters with photoelectric cells were introduced in the early 1930s. These used the photovoltaic effect of selenium metal. Exposed to light, selenium generates a voltage in proportion. This can be used to move a sensitive galvanometer mechanism swinging a needle to a position over a calibrated scale. No battery is required but these early meters are not particularly sensitive. Cadmium Sulphide (CdS) has a photo-resistive effect -it changes its resistance in proportion to light exposure. CdS meters need a battery to operate but are often more sensitive than selenium meters.
Before discussing how to interpret a light meter reading, let's first talk a little about how light meters work. There are two basic types, photovoltaic and photoresistant. a. Photovoltaic meters use a selenium cell, which converts light energy directly into an electric current. (It's the same kind of cell used to power solar calculators.) The current is used to make a needle move. The brighter the light, the stronger the current, and the stronger the current, the farther the needle moves. This is an extremely straightforward way of measuring light. Its chief advantage is that it needs no batteries, because the electric current is generated by the very same light the meter is measuring. So this type of meter can be used anywhere, anytime almost. Photovoltaic cells are not very efficient at converting light into electricity. In dim light, there just isn't sufficient current to move the needle enough to make an accurate measurement. This can be compensated for by making the selenium...
Another option, especially for those headed for a week or two in the back-country, is a solar-powered battery charger from ICP Global Technologies (www.icpglobal.com html pp.htm). This unit can charge four AA batteries in four hours or so, depending on how bright a day it is. (Good luck if you're hiking in the Pacific Northwest.) The hard part might be finding a way to keep the device pointed toward the sun while you're backpacking through the forest. You may also want to consider whether carrying the unit's weight (1.3 pounds) in extra batteries would be more efficient.
Light-sensitive selenium (Se) is the active element in a photovoltaic cell. Exposure to light generates an electric potential across the cell. A sensitive galvanometer in the circuit gives a deflection proportional to the incident light incident, and the necessary Light incident upon a solid-state device called a silicon (Si) photodiode (SPD) generates a very small current. Like the selenium cell this device is photovoltaic. But its output is too low to be used in the same way, and in cameras an amplifier is necessary to produce a useful output. An operational amplifier acts as a current-to-voltage converter, and with a suitable feedback resistance gives a high output voltage, linearly proportional to the incident light. The response is good even at very low light levels, and the linearity of response is maintained over a wide range of illuminance levels. The response time of an SPD is very short, of the order of microseconds, a useful property for switching functions. The cell area...
Solar powered chargers are available, but except on very long expeditions or very static trips, they will rarely be worth their weight, as you need a rest day with good weather to charge a single battery. Even if you do carry one in the wild, make sure you do not overly rely on it, and bring enough batteries to get through your trip.
Getting Started With Solar
Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.