<p>What is meant with print quality, what makes people perceive the quality of an image in a certain way? An inquiry was made about what the parameters are that strongly affect the perception of digital printed images. </p><p>A subjective test and some measurements make the basis for the thesis. The goal was to find a tool to predict perceived image quality when investigating the connections between the subjective test and the measurements. </p><p>Some suitable images were chosen, with a variety of motifs. A test panel consisting of people that are used to observe image quality answered questions about the perception of the quality. Measurements were made on a special test form to get information about the six different printers used in the investigation. </p><p>One of the discoveries was made when two images with the same colorful motif were compared. The first image got a much higher grade for general quality than the second image, even though the second image was printed with a printer that had a larger color gamut. The reason of this is that the first image consists of more saturated colors, and the second image has more details. The human eye perceives the more saturated image to be better than the image with more details. Another discovery was the correlation between the perceived general quality of a colored image and the perceived color gamut. One conclusion was that a great difference between two calculated color gamuts resulted in a large difference in perception of the color gamuts. A discovery of an image with very few colors and many glossy surfaces was that print mottle and sharpness are strictly connected to the general quality.</p>

This thesis seeks to gain a better understanding on operating characteristics of the three primary color light emitting diode (LED). By applying direct, pulse and sinusoidal currents with dimming function on red, green, and blue LEDs, respectively, the operating characteristics are investigated, including electrical characteristics and their effects on the light efficiency, spectral power distribution, chromaticity on each color LED and the resultant color gamut. The analysis reveals that the illumination characteristics intimately relate to the driving current. LEDs that are driven by pulse currents with pulse-width-modulation (PWM) dimming have less color shift than those driven by direct and sinusoidal currents with amplitude modulation dimming. However, the problematic color shifting is not acceptable when LEDs with pulse current are dimmed at a lower level. Based on the investigation results, a dimming scheme with PWM and pulse- amplitude-modulation (PAM) is proposed to correct the chromaticity and hence to improve the color gamut.

A high efficiency driver circuit is proposed for Light Emitting Diode (LED) lamps with dimming feature. The current regulation is accomplished by processing partial power of the power conversion circuit so that a high overall efficiency can be realized. The detailed description and analysis of circuit operation are provided. The dimming feature can be accomplished by means of linear current regulation, pulse-width modulation (PWM) or double pulse-width modulation (DPWM). Based on the circuit analyses and derived equations, a laboratory circuit is designed for an LED lamp which is composed of 40 high-brightness white LEDs in series. The performances with three dimming schemes are compared from the measured results. LEDs dimmed by DPWM have less color shift than those dimmed by linear current regulation and PWM. On the other hand, the dimming scheme with linear current regulation has the highest light efficiency over the entire dimming range. The circuit efficiency can be as high as 95.5% at the rated output and deteriorates slightly to 90.5% as the lamp is dimmed to 10% of the rated power.

What is meant with print quality, what makes people perceive the quality of an image in a certain way? An inquiry was made about what the parameters are that strongly affect the perception of digital printed images. A subjective test and some measurements make the basis for the thesis. The goal was to find a tool to predict perceived image quality when investigating the connections between the subjective test and the measurements. Some suitable images were chosen, with a variety of motifs. A test panel consisting of people that are used to observe image quality answered questions about the perception of the quality. Measurements were made on a special test form to get information about the six different printers used in the investigation. One of the discoveries was made when two images with the same colorful motif were compared. The first image got a much higher grade for general quality than the second image, even though the second image was printed with a printer that had a larger color gamut. The reason of this is that the first image consists of more saturated colors, and the second image has more details. The human eye perceives the more saturated image to be better than the image with more details. Another discovery was the correlation between the perceived general quality of a colored image and the perceived color gamut. One conclusion was that a great difference between two calculated color gamuts resulted in a large difference in perception of the color gamuts. A discovery of an image with very few colors and many glossy surfaces was that print mottle and sharpness are strictly connected to the general quality.

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