In photometry, the measurement of the power of light is carried out using two different instruments: a goniophotometer and an integrating sphere. Both have their own salient features which make them appropriate for different types of flux testing and measurement.
Although they are used for the same purpose i.e. optical power measurement, they have different characteristics and principles. While it is obvious that the mode of operation is different in these instruments, one thing to also note is the type of lamps (or the source of light) that are tested. Therein lies the chief difference between the two. Here’s a look at the eight such key differences between an integrating sphere and a goniophotometer.
Goniophotometer vs Integrating Sphere
Before getting down to the comparison, here’s a simple introduction to these instruments.
What is a Goniophotometer?
A goniophotometer is a photometric device that is used to measure the perceived power of a light source at different angles. It is mostly used to test the luminous flux of directed light sources such as LED and automotive headlights.
It is based on the principle of a photometer with a revolving arm that is fitted with a circular mirror. Light is continuously fed to this mirror through different angles (as the arm rotates) which provides information about the luminous flux, intensity distribution, and efficiency of the source.
What is an Integrating Sphere?
On the other hand, an integrating sphere, as the name suggests, is a sphere-like structure that is used to measure the power of undirected light sources. It works on the principle of diffusion where light enters the sphere through tiny holes, reflects through the interior coating of the sphere, and gets distributed equally inside. This allows for flux measurement and many more actions.
An integrating sphere is also known as an Ulbricht sphere and is sometimes interchangeably used as a Coblentz square. The latter has a mirror-like inner structure as opposed to the diffusive one in the integrating sphere. The inner coating of the sphere is the most important element in the measurement process.
Eight Differences Between a Goniophotometer and an Integrating Sphere
These differences range from their characteristics to measurement mode to cost to applications.
1. Measurement of Total Power
The biggest advantage of an integrating sphere over a goniophotometer is its ability to measure the full power of a light source in a single measurement process. There is no need to carry out iterations which are required if the latter is used.
This makes the integrating sphere a sought-after instrument in the world of photometry. The integrating spheres models offered by CI Systems are cutting edge and are perfect for industrial applications.
2. Accuracy Dependence
In an integrating sphere, as mentioned above, the accuracy is entirely dependent on the type of inner coating used. In goniophotometry, it is dependent upon the iterations as well as the number of points used. The aggregate mean of the iterations will give an approximate value.
3. Applications
Both instruments are used for light power measurement. But when it comes to the scattering of light and spatial distribution information, there is a difference between them.
A goniophotometer is primarily used for directed light sources. Unidirectional lights will have greater accuracy when measured through such a meter. An integrating sphere can provide better results for ambient light power measurement.
For example, if a person wants to check the power of light in a living room (where there are multiple sources of light i.e. ceiling lights, a table lamp, and Christmas lights), they will use an integrating sphere to capture all the sources inside the sphere and then test the overall luminosity. With a goniophotometer, this will not be possible.
This is why integrating spheres are preferred for testing of light sources in industrial and radiometric applications.
4. Cost Differences
Integrating spheres used to be generally expensive. In comparison, a goniophotometer is much more expensive as it involves special types of spatial mirrors. Moreover, the individual parts of such a meter are priced highly.
It is important to note that the instruments must always be chosen per their applications and not the price.
5. Color Uniformity Testing
An advantage that goniophotometer has over an integrating sphere is its functional ability to check color uniformity and temperature. When equipped with color sensors, these additional characteristics can be measured.
An integrating sphere also does not provide information about the light distribution and spatial quantities.
6. Different Types
An integrating sphere is more or less the same when it comes to types. It is available in different sizes and modes of operation (manual, automatic), but that’s about it.
A goniophotometer is primarily available in three different types: A, B, and C. This involves changes in the revolving ability of the axes. While type A has a fixed horizontal axis, types B and C have a fixed vertical axis. These types are used for different types of lamps.
For example, type C is used for spot and tube lamps.
7. Speed of Operation
This is a debatable topic but the consensus in the industry shows that an integrating sphere achieves the measurement faster than a goniophotometer. This is because the latter takes a lot of time to revolve its arm for a single iteration.
Although an integrating sphere requires an additional detector for the measurement, it gets the job done quickly.
8. Maintenance Aspect
In an integrating sphere, the coating can be damaged if the source of light is high-powered. This is an unavoidable aspect and the coating will have to be changed if the sphere is to be used again. This can be an expensive affair. Some of the materials used for this purpose are barium sulfate and magnesium oxide.
A goniophotometer is a high-maintenance device with a lot of moving parts. While these parts are available in the market, replacing or repairing them can be heavy on the wallet. Understanding the operation and choosing wisely then becomes important.
Conclusions
These are the primary eight differences between an integrating sphere and a goniophotometer. For a photometry expert choosing between the two is a matter of application. However, these factors will also help in understanding what the core differences are.
Both the systems have been around from the start of the twentieth century and over the decades they have undergone vast changes. This puts the onus on the engineers to buy these products from trusted manufacturers.
For more information on integrating spheres and associated instruments, you are welcome to contact us.