When it's ''lights up'' time on Broadway, Tharon Musser earns her share of applause for creating for the stage much of that special magic of theater.
As one of the country's best-known lighting designers, Miss Musser has illuminated over 200 shows on or bound for Broadway, including the current musical hits ''Dreamgirls,'' ''Chorus Line,'' and ''42nd Street.'' At the moment she is plotting the lighting effects for the show ''Merlin,'' which opens Dec. 19 in New York, and for Neil Simon's new play, ''Brighton Beach Memoirs,'' which arrives here in March, after a December opening in California.
Her design studio is on the third floor of the 19th-century Federal house she owns in Greenwich Village. It is there, among a colorful array of posters from the dozens of shows she has lighted, that the designer works ''crazy hours'' at her drafting board, developing the light plans that are so important to building stage illusions. Her work also involves determining the equipment that will be required for each show.
Tharon Musser considers herself one of those few who pioneered the field of theatrical lighting design for women. ''I came in,'' she says, ''when it was just beginning to be acknowledged as a legitimate and respectable component of staging.''
Today, she says, hundreds of schools around the country are turning out graduates with degrees in theater lighting. A few of them will make it to Broadway. Many will be absorbed into regional theater, opera, and ballet companies.
''The craft of stage lighting can be taught. It includes knowing about electricity and physics and optics, and students must learn that,'' she says. ''But you cannot teach the art that is beyond craft, nor the special talent that extends it.''
This extra dimension of art and talent have to do, she says, ''with having a genuine feel for theater, a developed sense of taste, a knowledge of what makes star quality. It involves knowing how to read a script and then how to project it mentally onto a stage.''
She herself is a native Virginian who discovered the world of theater at Berea College in Kentucky and then went on to the Yale Drama School. ''By the time I finished at Yale, I knew my love was lighting - not acting, not scenery, not costumes.''
What constitutes good stage lighting? ''Creating the right atmosphere for the words and actions of a particular theater piece to happen in,'' Miss Musser says. ''It is literally painting with light in such a way that it can help determine mood, locale, time of day, season, and geographical location. Good lighting helps focus audience attention for the director. It involves the playwright and what he has written, the costumes, the sets.''
Theatrical lighting has changed enormously. It has benefited most, she says, from the new electronic technology stemming from the space program and from the portable light and sound techniques that have been developed by rock 'n' roll musicians for their one-night stands.
''I have been fortunate to witness the birth of computer-memory boards on Broadway. These have opened up the possibilities enormously,'' she recalls. ''They have freed the way I can work and the effects I can get on the stage. It would, for instance, have been quite impossible to light the fast-paced 'Dreamgirls' - as audiences see it today -with the old manually operated systems. Now cues are programmed into the memory bank, which makes for far greater speed, scope, and consistency in the lighting for each performance.''
The first electronic light-control system was introduced in 1975 with the opening of ''Chorus Line'' on Broadway. Now, Miss Musser estimates, the lighting for almost all shows is programmed on computers. And each year better and more sophisticated equipment is developed.
Currently, when she is plotting the lighting plan for a Broadway musical, she determines placement of from 400 to 600 spotlights. Straight dramas require from 200 to 250 spotlights. She is most successful, she feels, when she works as part of a creative team from the inception of a new show. She particularly enjoys the creative team that includes director-choreographer Michael Bennett, set designer Robin Wagner, and costume designer Theoni Aldredge. ''The communication between us is so fine-tuned and complete that this incredible collaboration happens that results in shows like 'Chorus Line,' 'Ballroom,' and 'Dreamgirls.' We get a show that is 'of a piece' and has oneness.''
In all cases she works with a show, making adjustments and changes, right up to opening night, and is on hand for all out-of-town openings. Besides the many shows she has lighted on Broadway; off-Broadway; and in England, Europe, and South America, she has worked for repertory companies in many cities, including Dallas, Miami, and Boston.