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I recently got interested in electronic paper. One design that I liked was the one from Mirasol: https://en.wikipedia.org/wiki/Interferometric_modulator_display

enter image description here

However, what I don't understand, is the following: Their subpixels are either ON (color) or OFF (black). Therefore, I am wondering how they can produce lighter and darker colors?

By lighter and darker colors, I mean that the color intensity is tuned. For instance for red:

enter image description here

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  • $\begingroup$ From the article, it looks like the pixels can only be on or off. But that's OK for many types of displays (but not for applications like TV screens). Also, intermediate shades of colors can be visually shown through pixel "dithering" which is how newspapers are able to display the appearance of gray-scale pictures even if they only have black ink. $\endgroup$ – Samuel Weir Mar 16 '18 at 17:01
  • $\begingroup$ thanks a lot for your comment. Can you point me to any explanation of dithering for someone who is not from the field ? Thanks. $\endgroup$ – james Mar 16 '18 at 20:48
  • $\begingroup$ Very quick and simple description of dithering: webstyleguide.com/wsg2/graphics/dither.html $\endgroup$ – Samuel Weir Mar 16 '18 at 21:17
  • $\begingroup$ Note that one pixel has many sub pixels. $\endgroup$ – Jon Custer Mar 16 '18 at 22:10
  • $\begingroup$ @JonCuster - I didn't even look closely at the diagram to notice that. Yes, if those individual blocks represent sub pixels, then it looks like there is 14-bit control over the intensity of each of the three primary colors. As pixels go, though, that's pretty big pixel with 6x7=42 sub pixels in it. $\endgroup$ – Samuel Weir Mar 16 '18 at 22:48
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What I don't understand, is the following: Their subpixels are either ON (color) or OFF (black). Therefore, I am wondering how they can produce lighter and darker colors?

It is a front illuminated display, operating on ambient light. When the display is not updated no power is being used, similar to epaper.

In the off condition a subpixel is black. In the on condition a subpixel is fully (100%) reflective for its time interval, it is the frequency of the intervals the determines the intensity and the combination of the subpixels that determines the color of each pixel. (Source: Page: 33, Column 7-Rows 40 through 65 of the following Patent).

The link you provided, in reference 6, leads to the Patent US5835255A "Visible spectrum modulator arrays" which explains the details behind their operation:

Page: 34 Column 9-Row 20 - "The pulse stream on each line 168 has a duty cycle appropriate to achieve the proper percentages of reflectance for the three Sections of each tri-dipole. Referring to FIGS. 10A, 10B, and 10C, for example, pulse stream 170 has a period T and a 50% duty cycle. For the first 50% of each period T the input to bias source 164 is high and the corresponding output of source 164 is a high voltage. During this portion of the cycle dipole antenna 80 will reflect all received light having the dipole antenna's resonant wavelength. For the second 50% of the cycle the output of source 164 will be low and dipole antennas 80 will absorb the received light. In FIGS. 10B, 10C, pulse streams 172, 174 represent a 30% duty cycle and a 100% duty cycle respectively; with a 30% duty cycle the effective intensity of the light radiation of the dipole antennas of the section will be 30%; for a duty cycle of 100%, the effective intensity is 100%.

For example, if a particular pixel of the image is to be brown, the relative intensities required of the three red, 25 green, and blue sections 60, 62, 64 may be, respectively, 30, 40, and 10. The input signals to the bias sources 164, carried on lines 168, would then have duty cycles, respectively, of 30%, 40%, and 10%. An adjacent pixel which is to be a brown of the same hue but greater brightness might require duty cycles of 45%, 60%, and 15%.

Referring to FIG. 11, to accomplish the delivery of the pulse width modulated signals from circuitry 26 to the pixel circuit modules 46, each circuit module 46 in the row includes storage 180, 182 for two bits. The bit 1 storage elements 180 of the modules 46 in the row are connected together to create one long shift register with the pulse width modulated signals being passed along the row data line 184 from pixel to pixel. If, for example, the period of the modulated signals is 1 millisecond and there are ten different intensity levels, then an entire string of bits (representing the on or off state of the respective pixels in the row during the succeeding 1/20 millisecond) is shifted down the row every 1/10 millisecond. At the end of the initial 1/10 millisecond all of the bits in elements 180 are shifted to the associated elements 182 by a strobe Pulse on strobe line 186. The content of each element 182 is the input to the driver 188 for the appropriate one of the three colors of that pixel, which in turn drives the corresponding section 60, 62, 64 of the tri-dipole. The rate at which data is shifted along the shift registers is determined by the number of elements on a given row, the number of rows, the number of intensity levels, and the refresh rate of the entire array.

Figures 10A, B and C.

Figure 11

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  • $\begingroup$ Thanks a lot for your answer. So, to sum it up: They use pulse-width modulation, like for a diode, to obtain different intensities of colors. Is that correct ? $\endgroup$ – james Jul 27 '18 at 7:31
  • $\begingroup$ Yes, the duty cycle determines the intensity. $\endgroup$ – Rob Jul 29 '18 at 6:19

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