Process control sensing of toner coverage

1. A toner coverage sensing system for sensing toner particles printed onto a surface of a process element using an electrophotographic printing system, the printed toner particles including printed porous color toner particles, comprising:

an optical sensing system including:

an infrared radiation source that directs infrared radiation in an infrared wavelength band onto the printed toner particles on the surface of the process element; and

a diffused radiation detector for sensing infrared radiation scattered from the printed porous color toner particles on the surface of the process element, wherein the diffused radiation detector is oriented such that the sensed infrared radiation does not include specular reflections from the surface of the process element; and

a data processing system that determines a sensed toner coverage for the porous color toner particles on the surface of the process element responsive to the sensed scattered infrared radiation;

wherein the porous color toner particles absorb less than 5% of the radiation in the infrared wavelength band.

2. The toner coverage sensing system of claim 1 , wherein the infrared wavelength band has a peak wavelength in the range of 850-1050 nm.

3. The toner coverage sensing system of claim 1 , wherein the process element is a photoconductor element, an intermediate transfer element or a receiver medium.

4. The toner coverage sensing system of claim 1 , wherein the diffused radiation detector is positioned on a same side of the process element as the infrared radiation source to sense reflected infrared radiation.

5. The toner coverage sensing system of claim 1 , wherein the diffused radiation detector is positioned on an opposite side of the process element as the infrared radiation source to sense transmitted infrared radiation.

6. The toner coverage sensing system of claim 5 , wherein the sensed transmitted infrared radiation is sensed using an integrating sphere.

7. The toner coverage sensing system of claim 5 , wherein the sensed transmitted infrared radiation does not include specularly transmitted infrared radiation.

8. The toner coverage sensing system of claim 1 , wherein the porous color toner particles have a porosity of at least 10%.

9. The toner coverage sensing system of claim 1 , wherein the color toner particles have an aspect ratio in the range of 0.6 to 1.0.

10. The toner coverage sensing system of claim 1 , wherein the sensed scattered infrared radiation from the printed porous color toner particles is at least 20% higher than would be sensed for printed non-porous color toner particles having a same pigment and resin and a substantially equivalent particle geometry and toner coverage as the printed porous color toner particles.

11. The toner coverage sensing system of claim 1 , wherein the printed toner particles on the surface of the process element also include non-porous black toner particles, the non-porous black toner particles absorbing at least 20% of the radiation in the infrared wavelength band;

wherein the optical sensing system further includes a specular radiation detector oriented to sense infrared radiation specularly reflected from the surface of the process element; and

wherein the data processing system determines a sensed toner coverage for the non-porous black toner particles on the surface of the process element responsive to the sensed specularly reflected infrared radiation.

12. The toner coverage sensing system of claim 1 , wherein the printed toner particles are dry toner particles.