Location
Are they in the same country as you? It may not be immediately obvious from their website. Some may have a U.S. office, but are operating out of countries for which OFAC sanctions and ITAR restrictions apply.
Consultant vs. Agency
Is the company providing you with the qualifications of the person you will actually be working with and ensuring their availability or will they subcontract to whomever is available? Also, compare the rates. In my experience, agency markups can be as high as 100%. If your company requires you to select from a list of preferred vendors, see if you can add my company to that list.
Experience
Anyone can set up an LLC and create an optical consulting website with an impressive list of capabilities and photos. Keep in mind that a consultant can rarely show images of work they’ve done for their customers, because that IP doesn’t belong to them.
Take a look at the consultant’s LinkedIn profile. Ideally, they will have a graduate degree in optics and a decade or so of experience working with optical designs under the guidance of more experienced optical designers and analysts.
In addition to that, a well rounded optical designer will have a couple years of experience in optical systems (requirements development, trades, or project management).
Have they been consulting full time for over a decade, are they just starting out, or are they moonlighting? We all have to start somewhere, but if they’re relatively new to consulting, their rate and your schedule should reflect that.
Testimonials
Look for testimonials on their website and LinkedIn. If they’ve done a lot of consulting projects they should have at least a few if not a dozen.
Optical Design Tools and Capabilities
What design tools do they have? Do they have a Zemax software license and is it currently supported? Do they have the other software tools they’ll need for your project, such as Matlab for image processing?
Do they have an optical prototyping lab and the basic measurement tools they’ll need for your project (e.g., light sources, photodiodes, polarization elements, beam collimators, and resolution targets)?
Specialty
Do they have experience designing the type of optical system you need? If they meet the other criteria I’ve listed, they should be able to figure it out. That said, there is trade knowledge associated with most application areas that can’t just be looked up in a book. The only way to become proficient at designing microscopes, telescopes, spectrometers, AR glasses, machine vision cameras, flux concentration optics, and so on, is to do it.
If you can find an optical designer that specializes in your application, that may be your best bet. If not, hire someone with strong qualifications in optics, enthusiastic testimonials from clients, and a lot of experience.