Experimental methods

In this section, we want to find the best drive methods in white-light LED PWM operation and verify the theory described in Section 3.1 for the suitability of white-light LED lighting.

The experiment we report does not work in a dark environment. The environment is not a completely dark room. The room often had a lot of external ambient light. For the above reasons, we design a realistic lighting environment and we aimed to find the operation range that works.

There are three main experiments that will be conducted. They are flicker, CFF and stable lighting experiments. Each will aim to find the greatest sensitivity, the flicker fusion frequency, and the best stable lighting operation.

Before discussing the three experiments methods, the experimental equipment will be briefly described.

4.2.1. Experimental equipment

For the purposes of our experiments, we designed simple equipment which will allow us to observe the different drive methods in equipment and record the human vision senses. This equipment includes a box with one side opened and an LED light attached to the inner top side. The length and width of the box are equal. We pasted white paper inside the box to produce indirect lighting. This equipment is shown in Fig.

4.1, Fig. 4.2, Fig. 4.3 and Fig. 4.4.

Fig. 4.1 Experiment equipment

As Fig. 4.1, experiment equipment is two boxes to one group, one box for control group, other box for comparison group. Fig. 4.2 shows the one of box front view. Fig. 4.3 shows side view of the equipment. There is a light barrier attached to the light box so that observers do not look directly at the light source. The x, y and z variables indicate the length, width and height of the light box. Each of them is a length of 20cm. The A and B points indicate two observation points. Point A is at the edge of

LED lamp Light barrier

Fig. 4.2 Front view

the light box and point B is at a 50cm distance (d) from the edge of light box. Both A and B are at the center line of y and z.

Fig. 4.4 is the top view of the equipment. Here, one can see the LED lights being used as the experiment light source. The LED lights are installed on the top center of the light box. The experiment light source is a row of seven white-light LED with a total length of 6.5cm and a width of 0.5cm. As Fig. 4.2, the low-angle shot was deliberately used in order to show the location of LED lamp. However, during the experiment, observers will look from a level angle so that the light barrier will prevent them from looking directly at the LED lamps.

A B

x

z

d

Fig. 4.3 Experiment side view

A B

x y

d

Fig. 4.4 Experiment top view

Fig. 4.5 Test method, equipment and observer

Most lighting systems operate indoors in a low light or no light environments. In order to consider the degree of impact of the light environment, the two observations points (A and B) will be used for comparison. As Fig.4.5, Z and X are 20 cm, d is 50 cm, observation point A represents low environmental impact and observation point B represents high environmental impact.

Prior to the experiment, measurements of the illumination (in lux meter) were taken at point A and point B. These measurements provide the basic data for the light environment and the no light environment.

Table 4.1 Environment (Lux)

The measurements can be found in Table 4.1 above. The eyes of the observer will detect the light phenomena at observation points A and B. Spatial frequency effects will

A B

Lamp OFF

LED OFF 0 3

Lamp ON

LED OFF 100 142

Lamp OFF

LED DC ON 780 20

Lamp ON

LED DC ON 780 172

A B

d X

Z

In the test environment, the first environment (with interior lights) had a luminance of 142 lux. The second environment (without interior lights) had a luminance of 3 lux.

When the light is driven using a DC-source, the environment luminance is 780 Lux at point A. At point B, 50cm away from the light box, the measured luminance is 172 Lux as listed in Table 4.1.

4.2.2. Flicker experiments methods

In this experiment, we want to find the highest sensitivity for human vision in order to design optimal warning lighting systems. The purpose of flicker light is for warning rather than general purpose lighting. The experiments aim to find the best PWM parameters of frequency value and duty value which attract the attention of the human observer. The experiment methods are as follows:

a. Fixed duty, varied frequency to find the highest sensitivity frequency in different duty.

b. Based on highest sensitivity frequency, adjust duty to find the different intensities.

Because Broca-Sulzer Law and Bloch’s Law only discuss how lighting duty ON time affects sensitivity, never periodic lighting; the driven duty in the first experiment is set at 50% square-wave in order to find the highest sensitivity frequency. This frequency was then used to adjust duty and detect sensitivity. Based on how brightness affects sensitivity, we chose a fixed duty between 10% and 90%, and adjusted frequency while recording the visual senses of the observer. By doing this, we are able to find the highest sensitivity duty and corresponding frequency range.

4.2.3. CFF experiments methods

Before the stable lighting experiments, we must find the frequency at which the human eye no longer senses flicker. This experiment works for general lighting systems.

General lighting systems must provide stable light so that human vision is comfortable.

The aim of this experiment is to find the highest CFF frequency.

First, a fixed duty between 10% and 90% was established; the frequency was adjusted, and the adjusted frequency at which the observer no longer sensed flicker was recorded. Because the brightness of the light environment also affected the CFF frequency, experiments had to be done in different light environments as shown in Table 1 above. The experiments only used one light box.

By using the highest CFF, we are able to make sure that the observer never senses flicker in stable lighting experiments.

4.2.4. Stable lighting experiments methods

In the stable lighting experiments, we want to find a method which could reduce the drive current without reducing light sense. Based on the results of the CFF frequency experiments, this can be achieved by using two times CFF value as the drive frequency in the light box and eliminating brightness effect. Under these circumstances, the observer will not detect any flicker. Once the flicker sense is eliminated, we used two sets of the same light box which were independently operated. The LED lamps in each box used different PWM operations. The duty value was adjusted slowly in order to compare the different results of two test boxes. The observer’s interactive responses were recorded and compared. The experiments show that the varying duty values could not be detected.