Agents on print media

1. Apparatus for determining dryness of an agent on print media, the apparatus comprising:

an infrared prism including a surface, the surface having an outer side to contact a portion of the agent on the print media, the infrared prism having a refractive index that is higher than a refractive index of the agent;

a source of infrared radiation to direct infrared radiation into the infrared prism and onto an inner side of the surface;

a detector to generate a signal based on infrared radiation reflected by the inner side of the surface to indicate the dryness of the portion of the agent.

2. The apparatus of claim 1 , further comprising an agent application device to apply the agent to the print media, and wherein the agent application device is to control application of the agent to the print media based on the signal.

3. The apparatus of claim 1 , further comprising print media transport to transport the print media through the apparatus based on the signal.

4. The apparatus of claim 1 , wherein the agent comprises one of primer and print agent.

5. The apparatus of claim 1 , wherein the outer side of the surface of the infrared prism is to continuously contact the print media.

6. The apparatus of claim 1 , wherein the infrared prism is moveable such that the outer side of the surface of the infrared prism is to contact the print media when the detector is to generate the signal.

7. The apparatus of claim 1 , wherein the detector is to generate the signal based on intensity of infrared radiation of a predetermined wavenumber reflected by the inner side of the surface.

8. The apparatus of claim 7 , wherein the predetermined wavenumber is 3200-3600 cm −1 .

9. A system to determine a solvent level of a substance on a substrate, the system comprising:

an infrared-transparent article, wherein a surface of the article is to touch a part of the substance on the substrate, and wherein a refractive index of the solvent is lower than a refractive index of the article;

an infrared beam generator to generate an infrared beam and to direct the infrared beam into the article;

a sensor to receive a reflected infrared beam reflected internal to the article at the surface and to provide an indication of the solvent level of the part of the substance in response to the reflected infrared beam.

10. The system of claim 9 , wherein the substance comprises primer, and wherein the system comprises:

conveyance apparatus to convey the substrate;

a primer application station to apply the primer to the substrate; and

a print agent application station to apply print agent to the primer on the substrate;

wherein the system is to control one of the primer application station and the print agent application station in response to the indication.

11. The system of claim 10 , wherein the system is to control the primer application station in response to the indication.

12. The system of claim 9 , wherein the indication indicates an intensity of the reflected infrared beam at a selected wavenumber.

13. The system of claim 12 , wherein the wavenumber comprises 3200-3600 cm −1 .

14. The system of claim 9 , comprising a printer, wherein the substrate is a print media and the substance is a print agent.

15. A method of using the system of claim 9 , further comprising:

bringing the surface of the article into contact with the substrate, which is a print media bearing a print agent;

sliding the print media over the surface of the article;

with the sensor, determining a level of solvent in the print agent on the print media; and

controlling operation of the printer based on the determined level of solvent.

16. The system of claim 9 , wherein the sensor is to compare a signal indicative of the reflected infrared beam to a calibrated signal generated by the detector when no substrate or agent is in contact with the article.

17. The system of claim 9 , further comprising:

a print media transport to transport the substrate; and

a controller to control speed of the print media transport based on the indication of the solvent level on the substrate as output by the sensor.

18. Apparatus to determine a solvent content of agent on print media, the apparatus comprising:

a prism to contact the agent on the print media;

an infrared radiation producer to direct infrared radiation into the prism and onto an interface between the prism and the agent on the print media;

a sensor to sense infrared radiation reflected by the interface to determine the solvent content of the agent on the print media.

19. The printing device of claim 18 , wherein the sensor is to sense an intensity of the infrared radiation reflected by the interface at a particular wavelength to determine the solvent content of the agent on the print media.

20. The printing device of claim 18 , wherein the agent comprises print agent or primer.