Painful but important, "Ethnic Notions" airs dirty laundry for a good cause. The 1986 documentary surveys racist imagery in American popular culture from the antebellum South through World War II. It makes the material appalling and instructive.

Channels 32 and 26 will show "Ethnic Notions" at 10 tonight, just after the first installment of "Eyes on the Prize," the award-winning six-part history of the civil rights movement that is getting a worthwhile reprise.

Whereas "Eyes on the Prize" is a television milestone, "Ethnic Notions" is more of a footnote, but a valuable one. It's a collection of icons and artifacts that show how blacks were depicted in stories, songs, calendar and post card art, and films, and the way these hurtful caricatures were used to rationalize slavery and, later, more subtle forms of repression.

Abhorrent examples of old stereotypes flash by, and one is alarmed at the virulence in many of them, and then, finally, at the sheer numbers -- and this is only a sampler. Marlon T. Riggs, the writer, producer and director, did a fine job of making the points he wanted to make and of modulating the hour's visual texture.

Before Emancipation, blacks were usually depicted as happy, carefree simpletons -- a cruel contrast to the realities of plantation life. After the Civil War, the portrayals turned harsh and threatening; the shuffling darky became the menacing brute.

Minstrel shows started as whites playing fairy tale caricatures of blacks. Later, blacks themselves appeared in minstrel shows, but only if they "corked up." The legendary Bert Williams and his battle with blackface are recalled in a too-lengthy dramatic vignette by choreographer Leni Sloan, one of many trenchant contributors to the show's ongoing commentary.

Why the chronology stops at World War II isn't made clear. "Amos 'n' Andy," in its TV and radio versions, isn't considered, perhaps because it was itself the subject for a previous documentary and has been much discussed. Unfortunately, Riggs' script gets preachier as it goes on; it's at its best when analyzing why the stereotypes were malicious, rather than just saying that they were.

It also seems unfair for Riggs to do "then and now" cross-cutting between an old song by Ethel Waters and a speech by the Rev. Martin Luther King Jr., because Waters, who achieved great dignity in her singing and acting career, is thus made to look somehow culpable. Mostly, "Ethnic Notions" is interested not in affixing blame but in measuring effects.

The title is a bit flat, and considering such contemporary incidents as the recent flap over remarks by Jimmy (the Greek) Snyder, Riggs might have borrowed a title from an old "Project 20" documentary: "Not So Long Ago." One would like to think the absurd stereotypes seen here are part of the distant past, but it's not distant enough to be very encouraging. Indeed, Esther Rolle, who narrates, notes that "TV programs with a mammy figure in the household" survive even today. She herself starred as a maid in the mammy mold on "Maude," later spun off into "Good Times." This isn't mentioned. But it probably doesn't have to be.