That is changing. A new miniature-chip technology from Texas Instruments (TXN) called pico is making digital projectors truly portable, instead of merely luggable. For the past couple of weeks, I’ve been using two of the first pico-based projectors on the market, Dell‘s M109S and Optoma’s Pico PK-101.

The products are designed for different customers with different needs. Dell (DELL) positions the 13-ounce M109S as a notebook companion, best suited for work presentations. The four-ounce Optoma projector is designed more as an iPod or digital-camera accessory for watching movies and slide shows on the go.

The Dell M109S

Their portability requires compromises, most obviously in brightness and image resolution. The Dell and Optoma projectors, respectively, support 11 and 50 lumens — a standard measure of projector brightness. That’s far dimmer than top-notch projectors that offer several thousand lumens. So neither product excels in well-lighted rooms, where overhead and ambient lighting overpower their images. You can compensate somewhat for this weakness by placing the devices closer to the surfaces onto which they’re projecting — for example, a wall. But the darker the room you use, the better.

At about the size of a candy bar, the $399 Optoma device is the smaller of the two projectors and the one with the most intriguing possibilities for expanding the tiny screen sizes of mobile devices like the iPod.

It’s powered by a rechargeable battery that Optoma says lasts for an hour on full brightness or two hours on a power-saving setting (the projector comes with two batteries). The projector has a tiny speaker, but people who want decent sound will need to use headphones or external speakers.

In theory, the Optoma device is small enough to bring along on a camping trip to show a film on the side of a tent, or to a restaurant, where you could inflict a vacation slide show on dinner mates by projecting onto a napkin or tablecloth.

I tested it on a recent airplane flight by projecting an episode of “Mad Men” from an iPod touch onto the back of the seat in front of me. The seat was a dark blue with embossing on it, so it didn’t work very well. It’s best to project onto an unmarked, light-colored surface. The quality of the image was better when I lay in bed one night, projecting a video onto a white ceiling.

Even under the most favorable circumstances, however, I found the images from the Optoma projector very dark, muddling the outlines of characters and action on screen. Although Optoma says you can get up to a 60-inch image from the projector, 45 inches was about as big as I could make the image before it got too fuzzy.

Optoma says the projector will ship with an iPod-compatible connector cable when it goes on sale in the U.S. on Dec. 15, though the unit I tested didn’t come with one. I connected the device to my iPod touch using a $50 cable from Apple (AAPL).

Compared with the Optoma device, the $449 Dell M109S is a behemoth, yet it’s still only about the size of a short stack of drink coasters. Most projectors weigh at least a few pounds, if not more, which is big enough to make them a hassle to carry around. I barely noticed the Dell projector inside my laptop bag.

Unlike the Optoma projector, the Dell M109S has to be plugged into an electrical outlet to work. It comes with an unsightly set of connectors for plugging the projector into a video source, such as the VGA port found on most laptops and a composite video plug that is standard on DVD players. I was, however, able to plug my iPod touch into the Dell projector using the $50 Apple cable.

And unlike the Optoma, the Dell doesn’t have speakers. To get sound for a movie, you’ll need headphones or speakers, like those on a laptop.

Despite its extra bulk, the Dell M109S literally outshines the Optoma projector. It produces a bright image that I found very watchable, even if it wasn’t high-definition. I projected the movie “James and the Giant Peach” onto an interior wall of my house, creating an image that was about 7 feet, measured diagonally.

The Dell M109S includes a capability called keystone correction, a standard feature in most projectors that adjusts a projected image to give it the proper dimensions, rather than the trapezoidal shape that results when a projector is angled upward. The Optoma projector doesn’t have this feature. To get a normal rectangular movie image, I had to hold the projector level, toward the projection surface.

For business travelers who do presentations or for people who want to create a theater-like experience in a hotel room, vacation house or against a sheet in the backyard, the Dell projector would be a good fit. For now, the Optoma projector is a good idea that needs refinement.

• Email Nick.Wingfield@wsj.com. Walt Mossberg is on vacation.