LED Measurement System

As we know, there are many light sources which can be used for lighting system. In the past, when the electricity hasn’t been used for lighting technology, candle and torch were used for it. Thomas Alfa Edison started lighting technology by his invention, incandescent lamp. Then, fluorescent lamp technology has been being used. Now, the development of LED light has begun to replace fluorescent lamp and other light source. LED is light source whose its efficiency is very high because it converts directly from electricity to light.

As a light source, LED has properties as other light source, such as: Flux Luminous, Intensity, Luminance, Radiance Flux, etc. Measuring these properties needs instrument or equipment. The development instrument technology makes the measurement of Light properties (especially LED light properties) become easier. To measure LED light properties need at least spectrometer, integrating sphere, fiber optic, and software. Several companies has developed this kind of instrument in one system.

Spectrometer

Spectrometer is equipments that analyzed the wavelength of light. Light that comes to spectrometer will be refracted by prism or grating, so light will be separated based on its wavelength. This wavelength will be analyzed by CCD to be converted into electric signal.

Fiber optic

Fiber optic is used to be path of light. Fiber optic is put in front of light source, so light can enter to the fiber optic and go to spectrometer to be analyzed

.

Integrating Sphere

Integrating sphere is a hollow sphere which is coated by white color. The function of integrating sphere is to collect 

all of light emitted by light source.

PC and software

Electric signals from spectrometer are the information of light. To know what signal said, it needs process to be understood. This process is done by computer with compatible software. Spectrometer is usually completed by its software.

LED Junction temperature measurement using forward voltage method

LED performance highly dependence to junction temperature. Therefore, LED junction temperature is one of the important parameter in LED. Until now, there are two kinds of measurement method which can be used for determining LED Junction temperature, Forward voltage method and Shift wavelength method. This article will discuss about LED junction measurement temperature using forward voltage method.

Forward voltage method based on the temperature dependence of diode forward voltage. Since LED is diode, this dependence also occurs at LED. The simple explanation of this dependence is the difference temperature will result different forward voltage, although current flows are same. For example, let’s say LED junction temperature is 20 Celsius and current is 20 mA, the forward voltage 2 Volt. Then when the junction temperature increase, let’s say 40 Celsius, the forward voltage will decrease, may be 1.96 volt. This kind of relation can be used for determining junction temperature.

This method contains two steps, calibration measurement and actual measurement. The function of calibration measurement is to get the reference data. For calibration measurement, we need several equipments: oven, pulse generator, oscilloscope, thermocouple, resistor (if using pulse voltage generator).

LED is put and heated inside oven until certain temperature. Then it is injected by pulsed current using pulse generator. The current must be same with the operating current of the LED. Then, measure forward voltage of LED. This measurement is repeated again with different temperature by increasing set point temperature of oven. It can be done by choosing several temperatures, such as: 20, 40, 60,..,120 Celsius. By this way we will get the relation between temperature and forward voltage.

There are several important things that we have to consider:

1.        Duty cycle from pulse generator must be very low, such as 0.1%. By using very low duty cycle, so heat dissipated from LED can be neglected, thus oven temperature can be assumed is same with LED Junction temperature. Finally, we can establish relation between Junction temperature and forward voltage.
2.    Thermocouple is used to make sure the oven temperature. Thermocouple must be put close to the LED. 
3. To make sure that LED temperature is same with oven temperature, LED must be waited inside oven for while after oven has achieved set point temperature. It can be 30 minutes or longer. 
4. If there are differences between set point temperature and temperature shown by thermocouple, so choose the temperature which is shown by thermocouple. It is because thermocouple is put close to LED, while temperature sensor at oven system may be at the other place.

The data from calibration measurement can be drawn as a graph. This graph is usually very linear. We can do the linear regression.

Next step is actual measurement. At this measurement, LED is put at the operating place and supplied by operating current. It may be DC current. When the LED is supplied by DC current, its voltage will decrease and achieve stable condition after a while. So, at this measurement, LED must be waited until it achieves stable condition. Approximately it will take 2 minutes. After that we can measure the forward voltage of LED by voltmeter or multimeter.

This forward voltage can be used to determine junction temperature measurement. Data from calibration measurement is used as reference. For example, the voltage from actual measurement is 1.7 Volt. Then, we can search what temperature shown when the forward voltage is 1.7 Volt.

Spectrometer

Light is kind of electromagnetic wave. Visible light for example, this is an electromagnetic wave which has wavelength that can be sensed by human eye. Source of light may emit light not only one wavelength. White light consist many wavelength from 380 nm – 750 nm. The proof about this is when white light (such as sun light) is refracted by prism, we can see many color (from red to violet) as a result. Color of light depends on the wavelength, so we can conclude that sunlight consist many wavelength.

Spectrometer is an instrument which can be used for measure light properties. The variable can be measured by spectrometer usually is intensity. Electronic spectrometer consists of device which can convert from light to electricity, it is called CCD.

The steps inside electronic-spectrometer are started with light enter the spectrometer, then a device (Prism or grating) refracts the light based on its wavelength. For example white light will be refracted into all of visible light wavelength. These wavelengths will be converted into electric signal by CCD. This signal will be transmitted to PC to be processed. Finally we can see the spectrum of light on the PC’s monitor. Usually electronic spectrometer is completed with its software which can be used to measure in several units, such as Watt, Lumen, Lux, etc.

Spectrometer is used for many applications, Such as solution identification and LED Junction temperature Measurement.

Thermal Insulation

When we put hot water inside thermos, water will still hot although it has been several hours. This also happens when we put cold water inside thermos. After hours, water will still cold. How come?

Some may think how thermos knows that inside is hot or cold thus thermos keep the water hot and cold. The answer of this question is Thermal Insulation. As we know that heat is transferred spontaneously from higher temperature to lower temperature. But if between hot and cold temperature is blocked by thermal insulation, heat transfer will be minimized. Actually it is impossible to makes heat transfer rate is zero. It means that although we put thermal insulation between two temperatures, heat transfer still occurs, but very small. By this explanation, we can say that hot water inside thermos also becomes cold but it needs long time.

When hot water is put in room temperature, water will transfer heat to the surrounding whether by conduction, convection or radiation. But when hot water is inside thermos, heat transfer is minimized. Thermos wall is made by thermal insulation material which has very small coefficient conductivity. This cause heat transfer from water to surrounding is blocked. So the temperature of water will stay hot until long time (longer than we put without thermos).

It also happens when cold water is inside thermos. Heat transfer should be from outside thermos to inside thermos because temperature water inside thermos is colder than surrounding. But this heat transfer is minimized by thermal insulation material. Therefore temperature water inside thermos will be kept cold for long time.

Pascal Application

Car can be easily lifted by hydraulic lift in garage is the example application of Pascal law. Another example is disc brake. The principle of these things are Pascal Law, that the pressure between two surfaces are same, thus bigger surface will have bigger force.

Hydraulic lift (Halliday Resnick: Fundamental Of Physics)

As we can see at the picture, surface area A1 is smaller than A2. Based on Pascal law, pressure at A1 and A2 are same. Pressure is force divided by area, P=F/A, thus force needed to lift the car will be much smaller than the weight of car.
    

Small force F1 makes pressure at A1, this pressure is transmitted to A2. The pressure work at A2 makes force with upward direction. The force produced in A2 will be much bigger than force applied F1. This is why car whose weight is very heavy can be lifted easily.

The same principle is also used in brake system, in this case is disc brake system. When brake handle is pulled with certain force, it will transmit pressure to another surface at other edge, and makes force to clamp disk. This disc is attached to the wheel, since the rotation of disc is de-accelerated thus the speed of vehicle also becomes slower.

Laser Based Manufacturing

The application of laser has been applied in many technologies. One of them is Laser based manufacturing. This technology is combined laser and photopolymer phenomena. Photopolymer is the phenomenon that polymerization of material by energy photon. Photopolymer material such as resin is liquid. If this liquid is shot by photon energy with certain wavelength, the phase will be solid because of the polymerization. Laser is narrow beam of light with very high energy. By controlling the beam of laser to the photopolymer material, an object can created.

The selection of laser type and photopolymer material is very important. Laser can be classified by its power and wavelength. Resin also can be selected based on properties. They have to be matched each other. If they are not match, failure will occurs. For example, the power of laser is too high, it will burn the resin. If wavelength of laser is not matched with resin’s properties, photopolymer phenomenon does not occur.

Controlling laser beam can be done by XY Scanner. This equipment can be programmed to drive laser beam. Inside XY scanner, there are mirror which has function to reflect the laser beam. By driving mirror movement, laser beam can be controlled.

This technology can be applied with another technology such as 3D camera. By taking 3D picture then process the data from picture to coordinates, it can be an input for XY scanner, thus laser beam can create the object which has the same with object taken by 3D camera. This kind of concept may be applied for construction of the skull of human. Before doctor do a surgery, this replica of skull can be used for practice.

Supersonic and shock wave

When the speed of objet is faster than speed of sound, it is called supersonic. Usually, it can be said as mach. For example, if the speed of sound and object 340 m/s and 680 m/s respectively, so it is called mach 2.

When the object has speed faster than sound of speed, it will be followed by shock wave. What is shock wave?

Sound is wave, when source emit sound, it will spread to any direction (figure a). If the source moves with low speed, we will get Doppler effects. If the speed is increased it will be like figure c. When the speed of object is increased until faster than speed of sound, as we can see at figure d, shock wave is formed.

Shock wave (Giancoli: Physics Principles with Applications)

The analogy of this phenomenon is when boat makes waves because its speed is faster than water waves.When aeroplane approach the speed of sound, sound barrier is in front of it. To be faster than speed of sound, aeroplane needs extra thrust to through the sound barrier. Once sound barrier is broken by aeroplane, sonic boom is formed. Sonic boom is produced only when the aeroplane breaks the sound barrier. 

Buoyancy Force application

How does boat float on the water?

Boats which are very big and heavy can float on the water, but needles are drowned at water. Why?

Every material in fluid (such as air and water) has buoyancy force. The direction of buoyancy is always upward. It means if buoyancy force is bigger than weight of the object, it will float.

When we put empty plastic bottle on the water, it will floats. This is because of the buoyancy force is bigger than weight of the bottle. Actually, there is small portion of bottle that drowned, but it is the key how it can float. Pressure occurs inside water with any direction. This pressure increases as the depth of water. Force can be formed if there is a surface in the water. As we know, pressure is force divided by area.

Buoyancy forces are formed by this phenomenon. Archimides principle said that buoyancy force is directly proportional with drowned volume. Mathematically, Buoyancy force is: F = ρ . g . V, where ρ is the density of fluid (water), g is gravity, and V is volume which is drowned.

Boat can float on water based on this principle. Boat has hull to get buoyancy force and makes the boat float. So it is very important to keep hull safe. In the sea, corals are sometimes found in the sea. Hull can crash the coral and causes the hull leakage. This condition can causes boat drowned since hull is the source of buoyancy force.

How to measure fluid flow

When fluid flows in pipe, there are several ways how to measure its flow. Ultrasonic is the one of technology that can measure fluid flow inside pipe. The measurement principle using ultrasonic doesn’t affects fluid flow because it is non-contact measurement. The other ways to measure fluid flow are contact measurement. This kind of measurement will affect the fluid flow.

Contact measurement of fluid flows usually use the principle that convert from flow to different pressure. Venturi tube, orifice, and pitot tube are the example of fluid flow measurement using conversion flow to different pressure.

Venturi meter is pipe narrow constriction. Fluid flows from pipe whose diameter is bigger, then enter the pipe whose diameter is smaller. Based on continuity law, flow rate will be same but the velocity will be different. 

Q1=Q2

A1.v1 = A2 . v2

As we can see at the simple equation above, if area of pipe is smaller, velocity will increase. How to measure flow?

Based on Bernoulli principle, pressure in small diameter pipe is different wit big diameter. This difference is used to determine fluid flow inside pipe. By using Bernoulli principle the difference of pressure can be converted to different velocity between big pipe and small pipe. This velocity difference is then used to determine flow rate (volume rate). Moreover, it also can be used to determine flow rate (mass rate) by using relation between density and volume of the fluid.

Fiber optic

Fiber optic is often used in telecommunication network. Comparing with electrical cable, signal transmission by fiber optic is faster because it uses light as a signal. Besides that, it is also immune to electromagnetic interference, transmits longer distance, and higher bandwidth data rates. How can fiber optic be a path for light?

Light can travel from edge of fiber optic to another edge. The principle of fiber optic is total internal reflection. Before discuss about total internal reflection, let us review about light refraction. Light will be refracted when light through different medium which their refraction index value is different. For example, if light is emitted from inside the water with certain angle, then it will be refracted when comes out from water to the air. The phenomenon of light refraction is the angles are different. If refraction index of medium where the light comes from higher than refraction index of medium where light goes on, the angle will be bigger. If the initial angle is enlarged, there will be a critical angle, that is the angle when the light is not transmitted to the second medium. Moreover, if the angle is still enlarged, the light will be reflected as total internal reflection.

Fiber optic consists of core and cladding. Core has higher refraction index than cladding, then if light travel inside core with the angle more than critical angle, so there will be total internal reflection. By this phenomenon light can travel from one edge to another edge.