灯具 is a category term ('lighting' as a section in a furniture store), not a specific lamp. For one lamp use
灯 or
台灯 (table lamp),
吊灯 (pendant),
落地灯 (floor lamp).
具 means 'tool/utensil' and forms many category words:
家具 (furniture),
文具 (stationery),
餐具 (tableware).
